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#include <iostream>

using namespace std;

const int maxN = 500000;

int count[maxN + 5];

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

  int n, k;
  cin>>n>>k;

  int good = 0;
  int result = 0;
  int a;

  for(int i=1; i <= n; i++){
    cin>>a;
    if(good < k && count[a] == 0){
      result += (i - good - 1);
      good++;
    }
    count[a]++;
  }

  if(good == k)
    cout<<result<<"\n";
  else
    cout<<"-1\n";
}