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#include<iostream>
#include<set>

using namespace std;

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

    int n, k;
    cin>>n>>k;
    int currentVal;

    long seconds = 0, elCounter = 0;

    set<long> bottles;

    for(int i=0; i<n; i++)
    {   
        cin>>currentVal;
        if(bottles.find(currentVal) == bottles.end() && elCounter < k)
        {   
            bottles.insert(currentVal);
            seconds += (i - elCounter);
            elCounter++;
        }
    }
    if(elCounter == k) cout<<seconds;
    else cout<<"-1";
    return 0;
}