#include <iostream>
#include <algorithm>

using namespace std;

int main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    long long n, k, tab[500000], a, res = 0, diff = 0;
    cin >> n >> k;
    for (int i = 0; i < n; ++i) 
        tab[i] = 500000;
    for (int i = 0; i < n; ++i) {
        cin >> a;
        if (tab[a-1] == 500000) diff++;
        if (i < tab[a-1]) tab[a-1] = i;
    }
    if (diff < k) {
        cout << -1;
        return 0;
    }
    sort(tab, tab+n);
    for (int i = 0; i < k; ++i) {
        res += tab[i];
    }
    res -= (k*(k-1)/2);
    cout << res;
    return 0;
}