#include <bits/stdc++.h>
using namespace std;
bool checkbit(int x, int y) { return (x & (1 << y)); }
int setbit(int x, int y) { return (x ^ (1 << y)); }
const int dirs[4][2] = {{1, 0}, {0, 1}, {-1, 0}, {0, -1}};
const int mod = (1e9 + 7);
const int INF = (0x7fffffff);
const int maxn = 0;
int n;
multiset<char> s;
int main() {
  cin >> n;
  string st;
  cin >> st;
  if (n > ('z' - 'a' + 1)) {
    cout << "-1\n";
    return 0;
  }
  for (int i = (0); i < (st.size()); i++) s.insert(st[i]);
  int ans = 0;
  for (int ch = ('a'); ch < ('z' + 1); ch++) {
    if (s.count(ch) == 0) continue;
    ans += s.count(ch) - 1;
  }
  cout << ans;
  return 0;
}
