#include <bits/stdc++.h>
int main() {
  char str[100000];
  int arr[30];
  int i, length, j, sum = 0, k, count = 0, flag = 0;
  for (i = 0; i < 26; i++) arr[i] = 0;
  while (scanf("%d %s", &length, &str) != EOF) {
    if (length > 26)
      printf("-1\n");
    else {
      for (j = 0; j < length; j++) {
        k = str[j] - 'a';
        arr[k]++;
      }
      for (i = 0; i < 26; i++) {
        if (arr[i] > 0) arr[i] = arr[i] - 1;
        sum = sum + arr[i];
      }
      printf("%d\n", sum);
      sum = 0;
    }
  }
  return 0;
}
