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

int n, k, a, nextpos, doned;
long long int res;
bool done[500010];

int main() {
	cin >> n >> k;
	nextpos = 0;
	for (int i=0; i<n; i++) {
		cin >> a;
		if (!done[a] && doned < k) {
			res += i - nextpos;
			done[a] = true;
			nextpos++;
			doned++;
		}
	}
	if (doned < k) {
		cout << -1 << endl;
	} else{
		cout << res << endl;
	}
}