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#include <iostream>
#include <vector>

using namespace std;

typedef long long int lint;

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

    int n, k;
    cin >> n >> k;
    vector<bool> present(n+1);
    vector<int> first_appearances;
    
    for (int i = 0; i < n; ++i) {
        int ai; cin >> ai;
        if (!present[ai]) {
            present[ai] = true;
            first_appearances.push_back(i);
        }
    }

    if (k > first_appearances.size()) {
        cout << -1 << endl;
        return 0;
    }

    lint sum = 0;
    for (int j = 0; j < k; ++j) {
        int diff = first_appearances[j] - j;
        sum += diff;
    }
    cout << sum << endl;
    return 0;
}