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#include <bits/stdc++.h>
using namespace std;
using ll = long long;

bool used[500009];
int arr[500009];

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

    int n, k;
    cin >> n >> k;
    for(int i = 0; i < n; i++) {
        cin >> arr[i];
    }
    ll sum = 0;
    int pos = 0, x = 0;
    while(x < k and pos < n) {
        if(not used[arr[pos]]) {
            used[arr[pos]] = true;
            sum += (pos-x);
            x++;
        }
        pos++;
    }
    if(pos >= n and x < k)
        cout << "-1\n";
    else
        cout << sum << "\n";

    return 0;
}