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#include<cstdlib>
#include<vector>
#include<iostream>
#include<algorithm>
#include<set>

using namespace std;

int main(){

  ios_base::sync_with_stdio(0);

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

  set<int> q;

  int dest = k; int res = 0; int ok = 0;
  for (int i = 0; i < n; i++){
    int but; cin >> but;
//    cout << "set contains" << q.count(but) << " / " << q.size() << " :  ";
    if (!q.count(but) > 0 || ok == k) { 
      ok ++; 
      q.insert(but);
    }
    else {
      res += (dest - i); dest ++; 
    }
//    cout << i << " " << but << "  -> " << res << endl;
  }
  if (ok >= k) { cout << res << endl; }
  else { cout << "-1\n"; }

}