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#include <iostream>
#include<vector>
#define ll long long
using namespace std;

int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    int n,k; cin>>n>>k;
    vector<int>tab(n);
    for(int i = 0;i<n;++i){
        cin>>tab[i];
    }
    vector<bool>wys(n+1);
    ll odp=0; int found = 0;
    for(int i = 0;i<n;++i){
        if(!wys[tab[i]]){
            wys[tab[i]]=true;
            odp+=i; odp-=found;
            ++found;
        }
        if(found==k)break;
    }
    if(found<k)cout<<-1;
    else cout<<odp;
}