#include <bits/stdc++.h>

using namespace std;


int main() {
    int n, k, count = 0;
    long long res = 0;
    set <int> s;
    
    scanf("%d %d", &n, &k);
    
    for(int i = 0; i < n; ++i) {
        int v;
        scanf("%d", &v);
        if(s.find(v) == s.end()) {
            res += i - count;
            ++count;
            if(count == k) {
                printf("%lld\n", res);
                return 0;
            }
            s.insert(v);
        }
    }

    printf("-1\n");
    
    return 0;
}