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#include <iostream>
#include <unordered_set>
using namespace std;

int main() {
    long long n, k;
    cin >> n >> k;
    std::unordered_set<long long> brands;
    long long res = 0;
    for (int i = 0; i < n; ++i) {
        long long brand;
        cin >> brand;
        if (brands.find(brand) == end(brands)) {
            brands.insert(brand);
            res += i;
            if (size(brands) == k) break;
        }
    }
    if (size(brands) < k) {
        cout << -1;
        return 0;
    }
    cout << res - (k * (k - 1) / 2);
}