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#include <bits/stdc++.h> //Michał Kuśmirek
using namespace std;     //XIII LO Szczecin

#define FOR(i, a, b) for(int i = (a); i <= (b); ++i)
#define REP(i, n) for(int i = 0; i < (n); ++i)

using ll = long long;
using pii = pair<int, int>;

template<class T> int size(T && a) {
	return (int)(a.size());
}
ostream& operator << (ostream &os, string str) {
    for(char c : str) os << c;
    return os;
}
template <class A, class B> ostream& operator << (ostream &os, const pair<A, B> &p) {
	return os << '(' << p.first << "," << p.second << ')';
}
template <class T> auto operator << (ostream &os, T &&x) -> decltype(x.begin(), os) {
	os << '{';
	for(auto it = x.begin(); it != x.end(); ++it)
		os << *it << (it == prev(x.end()) ? "" : " ");
	return os << '}';
}
template <class T> ostream& operator << (ostream &os, vector<vector<T>> vec) {
    for(auto x : vec)
    	os << "\n  " << x;
    return os;
}
void dump() {}
template <class T, class... Args> void dump(T &&x, Args... args) {
    cerr << x << "; ";
    dump(args...);
}
#ifdef DEBUG
# define debug(x...) cerr << "[" #x "]: ", dump(x), cerr << '\n'
#else 
# define debug(...) 0
#endif

//--------------------------------------------------

int main() {
	ios_base::sync_with_stdio(0);
	cin.tie(0);

	int n, k; 
	cin >> n >> k;
	vector<int> a(n);
	REP(i, n) {
		cin >> a[i];
		--a[i];
	}

	vector<bool> in(n);
	vector<pair<int, int>> Q(n);
	REP(i, n)
		Q[i] = pair(i, a[i]);

	sort(Q.rbegin(), Q.rend());

	ll ans = 0;
	int swaps = 0;
	REP(i, k) {
		if(in[a[i - swaps]]) {
			while(!Q.empty() and in[Q.back().second])
				Q.pop_back();
			
			if(Q.empty()) {
				cout << "-1\n";
				return 0;
			}

			auto [pos, x] = Q.back();
			
			in[x] = true;
			ans += pos - i;
			++swaps;
		} else in[a[i - swaps]] = true;
	}

	cout << ans << '\n';
}