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#include <iostream>
#include <vector>

using namespace std;

long long solve(const vector<int>& marki, int k) {
    int n = marki.size();
    long long res = 0;
    vector<bool> taken(n + 1);

    int pos = 0;
    for(int i = 0; i < n; ++i) {
        if(!taken[marki[i]]) {
            taken[marki[i]] = true;
            res += i - pos;
            ++pos;
            if(pos == k) {
                break;
            }
        }
    }

    return (pos == k) ? res : -1;
}

int main() {
    ios_base::sync_with_stdio(0);
    int n, k;
    cin >> n >> k;
    vector<int> marki(n);
    for(int i = 0; i < n; ++i) {
        cin >> marki[i];
    }
    
    cout << solve(marki, k);

}