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#include <bits/stdc++.h>
#define FOR(i, n) for(int i = 0; i < (n); ++i)
#define REP(i, a, b) for(int i = (a); i < (b); ++i)
#define TRAV(i, a) for(auto & i : (a))
#define SZ(x) ((int)(x).size())
#define X first
#define Y second
#define PR std::pair
#define MP std::make_pair
typedef long long ll;
typedef std::pair<int, int> PII;
typedef std::vector<int> VI;

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

	int n, k;
	std::cin >> n >> k;
	VI A(n);
	TRAV(i, A) std::cin >> i;
	std::vector<bool> was(n+1);

	int need = k;
	ll ans = 0;

	FOR(i, n){
		if(was[A[i]]){
			ans += need;
		}else{
			need--;
			was[A[i]] = true;
			if(need == 0) break;
		}
	}

	if(need > 0) std::cout << -1 << "\n";
	else std::cout << ans << "\n";

	return 0;
}