//  Created by Michal Kowalski on 07/12/2021.
//

#include <iostream>
#include <set>

int N,K;

int main() {
    scanf("%d %d",&N,&K);
    std::set<int> S;
    int o = 0;
    long long steps = 0;
    for(int i=0;i<N;++i) {
        int a;
        scanf("%d", &a);
        // not found
        if (S.count(a) == 0) {
            S.insert(a);
            steps += (long long)i-o;
            ++o;
        }
        if (S.size() == K) {
            printf("%lld\n",steps);
            return 0;
        }
    }
    printf("-1\n");
    return 0;
}