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#include <bits/stdc++.h>

using namespace std;

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

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

    vector<bool> seen(n);

    int ctr = 0;
    long long res = 0;
    for (int i = 0; i < n; ++i) {
        int a;
        cin >> a;
        if (not seen[a]) {
            seen[a] = true;    
            res += i - ctr;
            ++ctr;

            if (ctr == k) {
                break;
            }
        }
    }

    cout << (ctr == k ? res : -1) << "\n";
}