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#include <bits/stdc++.h>
#include <unordered_set>
using namespace std;

const int N = int(5e5) + 99;

int n, x, k;
int a[N];

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cin >> n >> k;
    unordered_set<int> wines = {};

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

    int sum = 0;
    int collected = 0;
    for (int i = 0; i < n; i++) {
        if (wines.count(a[i]) == 0) {
            wines.insert(a[i]);
            sum += i - collected;
            collected++;
            if (collected >= k) {
                cout << sum << endl;
                return 0;
            }
        }
    }
    cout << -1 << endl;
}