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#include <iostream>

using namespace std;

const int MAX_N = 512 * 1024;
const int NO_ANSWER = -1;

bool f[MAX_N];

int main() {
	ios_base::sync_with_stdio(0);

	int u = 0;
	int c = 0;
	long long m = 0;
	int n, k;

	cin >> n >> k;

	int b;
	for (int i = 0; i < n; ++i) {
		cin >> b;
		if (f[b]) {
			++c;
		} else {
			f[b] = true;
			m += c;
			if (++u == k) {
				break;
			}
		}
	}

	cout << (u == k ? m : NO_ANSWER) << endl;

	return 0;
}