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#include <bits/stdc++.h>

using namespace std;

int main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);

    int n,m;
    cin>>n>>m;
    int out=0;
    vector<bool>zliczanie(n+1);
    int a;
    int k=0;

    for(int i=0;i<n;i++)
    {
        cin>>a;
        if(!zliczanie[a])
        {
            k++;
            out+=i;
            zliczanie[a]=1;
        }
    }

    if(k<m)
    {
        cout<<-1<<"\n";
        return 0;
    }
    m--;
    int p;
    if(m%2==0) p=(m+1)*(m/2);
    else p=(m+1)*(m/2) + (m+1)/2;

    cout<<out-p;
}