#include <iostream>
using namespace std;

int const N = 5e5 + 9;
int tab[N];
int odw[N];

void task(int n, int k) {
    int r = 0, m = 0, ans = 0;
    for (int i = 0; i < n; i++) {
        if (!odw[tab[i]]) {
            ans += i - m;
            odw[tab[i]] = 1;
            r++;
            m++;
        }
        if (r == k) break;
    }
    if (r < k) cout << -1;
    else cout << ans;
}

int main()
{
    int n, k;
    cin >> n >> k;
    for (int i = 0; i < n; i++) {
        cin >> tab[i];
    }
    task(n, k);
}