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#include <bits/stdc++.h>

using namespace std;

int cache[500001];

int main(){
    int n, k, temp;
    cin >> n >> k;
    long seconds = 0L;
    int iter = 0;
    for(int i = 0; i < n; ++i){
        if(iter >= k){
            break;
        }
        cin >> temp;
        if(cache[temp] == 0){
            seconds += (i - iter);
            ++iter;
            cache[temp] = 1;
        }
    }
    if(iter < k){
        cout << -1 << '\n';
    }
    else{
        cout << seconds << '\n';
    }
    return 0;
}