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

using namespace std;

int main() {
    int n {};
    int k {};
    int a {};
    long long res {};
    vector<int> b(500000, 0);
    cin >> n >> k;
    for (int i=0; i<n; i++) {
        cin >> a;
        if (b[a-1] == 0) {
            b[a-1] = 1;
            k--;
        } else {
            res += k;
        }
        if (k == 0) {
            cout << res << endl;
            return 0;
        }
    }
    cout << -1 << endl;
    return 0;
}