#include <cstdio>

const int max_n = 500100;
int n, k;
int seen[max_n];

int main() {
  scanf("%d %d", &n, &k);
  for (int i = 0; i < n; ++i) seen[i] = 0;
  int non_unique = 0;
  int unique = 0;
  long long cost = 0;
  for (int i = 0; i < n; ++i) {
    int a = 0;
    scanf("%d", &a);
    --a;
    if (seen[a] == 0) {
      cost += non_unique;
      ++unique;
      seen[a] = 1;
    } else {
      ++non_unique;
    }
    if (unique == k) {
      printf("%lld\n", cost);
      return 0;
    }
  }
  printf("-1\n");
  return 0;
}