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#include <bits/stdc++.h>
using namespace std;

#define pb push_back
#define eb emplace_back
#define mp make_pair
#define ll long long
#define ull unsigned long long
//#define int LL
#define FOR(i,a,b) for(int i = (a); i <= (b); i++)
#define REP(i,n) FOR(i,0,(int)(n)-1)
#define VI vector<int>
#define VVI vector<VI>
#define PII pair<int, int>
#define PLL pair<ll, ll>
#define st first
#define nd second
#define ALL(x) (x).begin(), (x).end()
#define SZ(x) ((int)x.size())

template<class C> void mini(C& x, C y) { x = min(x,y); }
template<class C> void maxi(C& x, C y) { x = max(x,y); }

template<class TH> void _dbg(const char *sdbg, TH h){ cerr<<sdbg<<'='<<h<<endl; }
template<class TH, class... TA> void _dbg(const char *sdbg, TH h, TA... a) {
	while(*sdbg!=',') cerr<<*sdbg++;
	cerr<<'='<<h<<','; _dbg(sdbg+1, a...);
}

template<class T> ostream &operator<<(ostream& os, vector<T> V) {
	os << "["; for (auto vv : V) os << vv << ","; return os << "]";
}
template<class L, class R> ostream &operator<<(ostream &os, pair<L,R> P) {
	return os << "(" << P.st << "," << P.nd << ")";
}

#ifdef LOCAL
#define debug(...) _dbg(#__VA_ARGS__, __VA_ARGS__)
#else
#define debug(...) (__VA_ARGS__)
#define cerr if(0)cout
#endif

int n, k;
const int MAXN = 5e5 + 100;
int a[MAXN];
int cnt[MAXN];
VI before_k, after_k;
ll res;

signed main() {
	ios_base::sync_with_stdio(false); cin.tie(nullptr);

	cin >> n >> k;
	REP(i, n) { 
		cin >> a[i];
		cnt[a[i]]++;

		if(cnt[a[i]] > 1 and i < k) {
			before_k.pb(i);
		}

		if(cnt[a[i]] == 1 and i >= k) {
			after_k.pb(i);
		}
	}

	if(SZ(after_k) < SZ(before_k)) {
		cout << -1;
		return 0;
	}

	reverse(ALL(before_k));

	int cur = 0;
	REP(i, SZ(before_k)) {
		res += (ll)after_k[i] - k - cur;
		res += (ll)k-1 - before_k[i] - cur;
		cur++;
	}

	res += (ll)SZ(before_k) * (ll)SZ(before_k);

	cout << res;

	return 0;
}