#include <bits/stdc++.h>
using namespace std;
const int N = 1e5 + 1;
int n, freq[26];
char s[N];
int main(int argc, char **argv) {
  scanf("%d%s", &n, s);
  for (int i = 0; i < n; ++i) ++freq[s[i] - 'a'];
  int bad = 0, can = 0;
  for (int i = 0; i < 26; ++i) {
    if (freq[i] > 1)
      bad += freq[i] - 1;
    else if (freq[i] == 0)
      ++can;
  }
  if (bad > can)
    puts("-1");
  else
    printf("%d\n", bad);
  return 0;
}
