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#include <iostream>

constexpr int maxN = 5e5;
constexpr int maxK = 5e5;
constexpr int maxA = 5e5;

int n, k;
int a;
bool arr[maxA + 1];
int diff;
long long ans;

int main() {
    std::ios_base::sync_with_stdio(false);
    std::cin.tie(NULL);
    std::cout.tie(NULL);

    std::cin >> n >> k;

    for(int i=0; i<n; i++) {
        std::cin >> a;

        if(arr[a] == false) {
            arr[a] = true;

            ans += (i - diff);
            diff++;
        }

        if(diff == k) {
            std::cout << ans;
            return 0;
        }
    }

    std::cout << -1;
}