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#include <iostream>
#include <set>

using namespace std;

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

    int n, k;

    cin >> n >> k;
    int shelf[n];

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

    set<int> is;

    int result = 0;
    int i = 0; int j = 0;

    while (i < k && j < n) {
        if (is.find(shelf[j]) == is.end()) {
            result += j-i;
            is.insert(shelf[j]);
            i++;
            j++;
        }
        else {
            j++;
        }
    }

    if (i == k) {
        cout << result;
    }
    else {
        cout << -1;
    }
}