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#include <stdio.h>
#include <iostream>
#include <utility>

using namespace std;

int main() {
  int n, k;
  scanf("%d %d", &n, &k);
  int data[n+1];
  int pos[k];
  int dif[n+1];

  for (int i = 0; i < n; i++) {
    dif[i] = 0;
  }

  for (int i = 0; i < n; i++) {
    scanf("%d", &data[i]);
  }
  int counter = 0;
  int it = 0;
  while(counter < k && it < n) {
    if (!dif[data[it]]++) {
      pos[counter++] = it;
    }
    it++;
  }

  if (counter < k) {
    printf("-1");
    return 0;
  }

  long long sum = 0;
  for (int i = 0; i < k; i++) {
    sum += pos[i] - i;
  }
  printf("%lld", sum);


  return 0;
}