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#include <iostream>

const int N = 500000;

int brands[N];
bool brandSet[N+1];

int n, k;
int64_t total = 0;
int different = 0;

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

    std::cin >> n >> k;

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

    for(int i = 0; i < n && different < k; i++){
        if(!brandSet[brands[i]]){
            brandSet[brands[i]] = true;
            total += i - different;
            different++;
            if(different == k){
                std::cout << total;
                return 0;
            }
        }
    }

    std::cout << -1;

    return 0;
}