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#include <iostream>
#include <queue>
using namespace std;

int main() {
  long long n, k, c = 0, q, u[500005] = {0}, r = 0;
  cin>>n>>k;
  for (long long i = 0; i < n; ++i) {
    cin>>q;
    if (u[q] == 0) {
      u[q] = 1;
      ++c;
      r += i - c + 1;
      if (c == k) break;
    }
  }
  cout<<(c < k ? -1 : r)<<endl;
}