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

#define boost                     \
    ios_base::sync_with_stdio(0); \
    cin.tie(0);                   \
    cout.tie(0)


int main() {
    boost;
    int n, k; cin >> n >> k;
    vector<bool> seen(n+1, false);

    int64_t res = 0;
    int done = 0;

    for (int pos = 0; pos < n && done < k; ++pos) {
        int a; cin >> a;
        if (!seen[a]) {
            seen[a] = true;
            res += pos - done;
            ++done;
        }
    }

    cout << ((done < k) ? -1 : res) << '\n';

    return 0;
}