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#include <bits/stdc++.h>
#include <ext/numeric>
#include <unistd.h>
using namespace std;
using namespace rel_ops;
using namespace __gnu_cxx;

typedef long long ll;
typedef unsigned long long ull;
typedef unsigned uint;

#define ALL(c)          (c).begin(), (c).end()
#define REP(i, n)       for (int i = 0, $##i = (n); i < $##i; ++i)
#define FOR(i, b, e)    for (int i = (b), $##i = (e); i <= $##i; ++i)
#define FORD(i, b, e)   for (int i = (b), $##i = (e); i >= $##i; --i)
#define EACH(it, cont)  for (auto &it : cont)
#define ever ( ; ; )
#define skip continue
const int inf = 1000000001;

struct GetSizeHelper {
	int s;
	operator int () const { return s; }
	template<class T> GetSizeHelper(const T &t) : s((int) t.size()) {}
};
#define SIZE (GetSizeHelper)

template<class T> int unique(T &t, bool sorted = false) {
	if (!sorted) { sort(ALL(t)); }
	t.resize(unique(ALL(t)) - t.begin()); 
	return SIZE t;
}
template <class T, class U> bool remin(T &a, const U &b) { return b < a ? a = b, true : false; }
template <class T, class U> bool remax(T &a, const U &b) { return b > a ? a = b, true : false; }
template<class T> T fromString(const string &s) { T t; istringstream(s) >> t; return t; }
template<class T> string toString(const T &t) { ostringstream oss; oss << t; return oss.str(); }
struct StdinReader {
	StdinReader() { ios::sync_with_stdio(0); cin.tie(NULL); }
} stdinReader;
#define get stdinReader,

template<class T> StdinReader &operator, (StdinReader &sr, T &t) { cin >> t; return sr; }

template<class T> StdinReader &operator, (StdinReader &sr, vector<T> &v) {
	for (auto &x : v) { sr, x; } return sr;
}
#define dump     fancy_dumper::DumpFirst<&std::cerr>(),
#define say      fancy_dumper::DumpFirst<&std::cout>(),
#define $dump    fancy_dumper::Dumper<&std::cerr>
#define $say     fancy_dumper::Dumper<&std::cout>
#ifndef $
#define $(x) #x, fancy_dumper::ToggleSeparator(), (x),\
                 fancy_dumper::ToggleSeparator()
#endif

namespace fancy_dumper {

struct ToggleSeparator { };

template<ostream *Out>
struct DumpFirst { };

template<ostream *Out>
struct Dumper {
	static void *&last() {
		static void *p;
		return p;
	}
	static const char *&separator() {
		static const char *s = " ";
		return s;
	}
	static const char *&separatorNewLine() {
		static const char *s = "\n";
		return s;
	}
	static const char *&unseparator() {
		static const char *s = "=";
		return s;
	}
	Dumper() {
		last() = this;
	}
	~Dumper() {
		if (this == last()) {
			*Out << separatorNewLine();
			last() = 0;
		}
	}
	Dumper operator, (ToggleSeparator) {
		std::swap(separator(), unseparator());
		return Dumper();
	}
};

template<ostream *Out, class T>
Dumper<Out> operator,(Dumper<Out>, const T &e) {
	*Out << Dumper<Out>::separator() << e;
	return Dumper<Out>();
}

template<ostream *Out, class T>
Dumper<Out> operator, (DumpFirst<Out>, const T &e) {
	*Out << e;
	return Dumper<Out>();
}

}

template<class T, class Y>
ostream &operator<< (ostream &o, const pair<T, Y> &p) {
	return o << p.first << ' ' << p.second;
}

template<class T>
ostream &operator<< (ostream &o, const vector<T> &v) {
	bool isBegin = true;
	EACH (e, v) {
		if (!isBegin) { o << ' '; }
		isBegin = false;
		o << e;
	}
	return o;
}

template<class T>
ostream &operator<< (ostream &o, const set<T> &v) {
	bool isBegin = true;
	EACH (e, v) {
		if (!isBegin) { o << ' '; }
		isBegin = false;
		o << e;
	}
	return o;
}

template<class T>
ostream &operator,(ostream &o, const T &t) {
	return o << t;
}
// code starts here



int bin_search(int b, int e, const vector<int> &a, int x) {
	while (b < e) {
		int h = b + (e-b)/2;
		if (a[h] < x) {
			b = h+1;
		} else {
			e = h;
		}
	}
	return b;
}


struct Solution {
	vector<vector<int>> C, P, CM;
	int clen;
	
	void run() {
		int n, m;
		get n;
		vector<int> g(n), PC(n, -7);
		get g, m;
		string in;
		in.reserve(m+1);
		get in;
		
		// detect subcycles
		const int cnum = __gcd(n, m);
		C.resize(cnum);
		clen = m / cnum;
		//~ dump $(n), $(m), $(cnum), $(clen);
		REP (i, cnum) {
			C[i].resize(clen);
			REP (j, clen) {
				const int idx = (i + (ll) n * j) % m;
				const char ch = in[idx];
				//~ dump $(idx), $(ch);
				C[i][j] = (ch & 1 /*W*/) ? 1 : -1;
				if (idx < n) PC[idx] = j;
			}
		}
		
		// calc prefix sums
		P.resize(cnum);
		REP (j, cnum) {
			P[j].resize(2*clen+1);
			FOR (i, 1, clen) {
				P[j][i] = P[j][i-1] + C[j][i-1];
			}
			FOR (i, clen+1, clen*2) {
				P[j][i] = P[j][i-1] + C[j][i-1-clen];
			}
		}
		CM.resize(cnum);
		REP (i, cnum) calcCmin(i);
		
		// search for die-time of each player
		// 10^18 will fit max possible answer
		const ll llinf = ll(inf) * inf;
		ll best = llinf;
		REP (p, n) {
			const int c = p % cnum;
			//~ const int pc = (p % cnum + p / cnum) % clen;
			//~ const int pc = ll((p-c) / cnum) * n % clen;
			//~ const int pc = (p + (ll) n * c) % clen;
			const int pc = PC[p % m];
			const int csum = sum(c, 0, clen);
			//~ dump	$(p) , $(c), $(pc), $(csum);
			
			if (csum >= 0) {
				const int s = search(c, pc, g[p]); if (s) { remin(best, calc(s, p, n)); }
			} else {
				const int D = CM[c][pc];
				//~ dump $(D), $(g[p]);
				if (g[p] + D <= 0) {
					const int s = search(c, pc, g[p]);
					if (s) {
						//~ dump $(calc(s, p, n)), $(s), $(pc), $(n), $(p);
						remin(best, calc(s, p, n));
					}
				} else {
					// need more than one cycle
					int x = (g[p] + D) / -csum;
					if ((g[p] + D) % -csum) {
						++x;
					}
					ll ans = (ll) clen * n * x;
					g[p] += csum * x;
					const int s = search(c, pc, g[p]);
					//~ dump $(x), $(s), $(g[p]), $(ans), $(calc(s,p,n));
					remin(best, calc(s, p, n) + ans);
				}
			}
		}
		if (best == llinf) {
			say "-1";
		} else {
			say best;
		}
	}
	
	int search(int c, int pc, int gp) const {
		FOR (k, 1, clen) {
			//~ if (pc==2015&&abs(k-89547)<10)//~ dump $(k),$(c), $(pc), $(gp), $(sum(c,pc,pc+k));
			//~ dump $(k),$(c), $(pc), $(gp), $(sum(c,pc,pc+k));
			if (sum(c, pc, pc + k) + gp <= 0) {
				return k;
			}
		}
		return 0;
	}
	
	void calcCmin(int c) {
		if (sum(c, 0, clen) >= 0) {
			// no need
			return;
		}
		
		CM[c].resize(clen);
		int s = 0, cmin = 0;
		REP (k, clen) {
			s += C[c][k];
			remin(cmin, s);
		}
		
		CM[c][0] = cmin;
		REP (k, clen-1) {
			cmin -= C[c][k];
			//~ //~ dump $(C[c][k+1]-C[c][k]), $(k), $(s), $(cmin);
			remin(cmin, s);
			CM[c][k+1] = cmin;
		}
		//~ dump $(c), $(C[c]), $(CM[c]), $(P[c]);
	}
	
	int sum(int c, int b, int e) const { return P[c][e] - P[c][b]; }
	
	// player p will die after `s' his steps
	ll calc(int s, int p, int n) const {
		return 1 + p + ll(s-1) * n;
	}
};

int main() {
	Solution s;
	s.run();
}