#include <iostream>
#include <set>
#include <utility>
#include <vector>

using namespace std;

int main() {
    int n, k;
    cin >> n >> k;
    int tab[n];
    vector<pair<int, int>> s;
    set<int> s2;

    for (int i = 0; i < n; i++) {
        cin >> tab[i];
        if (s2.find(tab[i]) == s2.end()) {
        s2.insert(tab[i]);
        s.push_back({i, tab[i]});    
        }
    }

if (s.size() < k) {
    cout << -1 << endl;
    return 0;
} else {
    int w = 0;
    for(int i = 0; i < k; i++) {
        w += (s[i].first - i);
    }
    cout << w << endl;
}

    return 0;
}