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#include <iostream>

const long MAX = 500000;
bool O[MAX];

int main() {
  std::ios_base::sync_with_stdio(0);
  long n,k,l=0;
  std::cin >> n >> k;
  long r = 0;
  for (long i=0;i<n;++i) {
    long a;
    std::cin >> a;
    if (l >= k) continue;
    if (O[a]) continue;
    O[a] = true;
    r += i-l;
    ++l;
  }
  if (l != k) r = -1;
  std::cout << r;
  return 0;
}