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#include<bits/stdc++.h>
using namespace std;

int main() {

    ios_base::sync_with_stdio(0);
    long long n,q;
    cin >> n >> q;
    vector<bool> is(n+1,false);
    vector<long long> vec;
    for(long long k=0;k<n;k++) {
        long long t;
        cin >> t;
        if(!is[t]) {
            is[t] = true;
            vec.push_back(k);
        }
    }
    if(q > vec.size())
        cout << "-1";
    else {

        long long res = 0;
        for(long long k=0;k<q;k++)
            res += vec[k] - k;
        cout <<res;
    }

}