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#include <iostream>
#include <set>
#include <algorithm>

using namespace std;

int main()
{
    ios_base::sync_with_stdio(false);
    long long wyn=0;
    int n, k;
    cin>>n>>k;
    set<int> w;
    for (int i=0; i<n; ++i)
    {
        int a;
        cin>>a;
        if (w.insert(a).second)
            wyn+=i+1-w.size();
        if (k<=w.size())
        {
            cout<<wyn<<endl;
            return 0;
        }

    }
    cout<<-1<<endl;
    return 0;
}