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#include<iostream>
#include<vector>
#include<algorithm>

using namespace std;

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

	int n, k;
	cin >> n >> k;

	vector<int> v(n);
	vector<bool> b(n + 1, false);

	for (int i = 0; i < n; i++) {
		cin >> v[i];
	}
	int p = 1, i = 1;
	b[v[0]] = true;
	int res = 0;

	while (i < n && p < k) {
		if (b[v[i]] == false) {
			res += i - p;
			p++;
			b[v[i]] = true;
		}
		i++;
	}
	if (p == k) cout << res;
	else cout << "-1";
}