#include <bits/stdc++.h>

using namespace std;

const int N = 5e5 + 7;

int oranz[N];

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

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

    long long res = 0;
    int tmp = 0;

    for (int i = 0; i < n; ++i)
    {
        int a;
        cin >> a;

        if (!oranz[a])
        {
            ++tmp;
            oranz[a] = 1;
        }
        else
        {
            res += k - tmp;
        }

        if (tmp == k)
            break;
    }

    if (tmp < k)
        cout << "-1\n";
    else
        cout << res << "\n";

    return 0;
}