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

set<int> s;

int main()
{
    int n,k, bottle;
    long long result=0;
    cin>>n>>k;
    long long originalK = k;
    for(int i=1;i<=n;i++)
    {
        cin>>bottle;
        if(s.find(bottle)==s.end())
        {
            s.insert(bottle);
            k--;
            result+=i;
        }   
        if(k==0)
        {
            cout<<result - (originalK*(originalK+1)/2)<<endl;
            return 0;
        }
    }
    cout<<-1<<endl;
    return 0;
}