#include <bits/stdc++.h>
using namespace std;
const double PI = acos(-1);
const int maxm = 2000 + 5;
const int mod = 1e9 + 7;
const int inf = 0x3f3f3f3f;
const long long INF = 0x3f3f3f3f3f3f3f3fLL;
const double eps = 1e-8;
const int maxn = 4e6 + 10;
int n;
int l, r;
string s;
string sj[maxn], so[maxn];
int main() {
  scanf("%d", &n);
  cin >> s;
  int len = s.size();
  int cnt = 0, ans = 0;
  sort(s.begin(), s.end());
  for (int i = 0; i < len; i++) {
    if (s[i] == s[i + 1]) {
      so[cnt++] = s[i];
      i++;
    } else {
      sj[ans++] = s[i];
    }
  }
  if (ans != 0)
    cout << ans << endl;
  else
    cout << 1 << endl;
  return 0;
}
