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#include <bits/stdc++.h>
#define rep(i, a, b) for (int i = a; i <= b; i++)
#define per(i, a, b) for (int i = b; a <= i; i--)
#define cat(x) cerr << #x << " = " << x << '\n';
using ll = long long;
using namespace std;

const int N = 500500;

int n, k, vis[N];

int main() {
	cin.tie(0)->sync_with_stdio(0);

	cin >> n >> k;

	ll res = -1ll * k * (k + 1) / 2;
	rep(i, 1, n) {
		int x;
		cin >> x;
		if (!vis[x] && k) {
			vis[x] = 1;
			k--;
			res += i;
		}
	}

	if (k) {
		res = -1;
	}

	cout << res << '\n';
	return 0;
}