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#include<bits/stdc++.h>

using namespace std;

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    long long n, k;
    cin >> n >> k;
    vector<int> bottles(n+1);
    vector<bool> taken(n+1, false);
    for(int i=1; i<=n; i++)
        cin >> bottles[i];
    long long sum=0;
    int counter=0;
    for(int i=1; i<=n; i++)
    {
        if(!taken[bottles[i]])
        {
            sum+=i;
            taken[bottles[i]]=true;
            if(++counter==k)
                break;
        }
    }
    if(counter<k)
    {
        cout << -1;
    }else{
        sum -=((k*(k+1))/2);
        cout << sum;
    }
    return 0;
}