#include <iostream>
#include <unordered_set>

using namespace std;

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

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

    long long res = 0;
    unordered_set<int> S;
    for (int i = 1; i <= n; i++) {
        int a;
        cin >> a;
        if (S.insert(a).second)
            res += (long long)i - S.size();

        if (S.size() == k) {
            cout << res;
            return 0;
        }
    }

    cout << -1;
}