#include <bits/stdc++.h>
using namespace std;
const int N = 1000007, inf = 0x3f3f3f3f;
int f[507][507][28];
string s;
int solve(int l, int r, char c) {
  if (l == r) return 0;
  int& st = f[l][r][c - 'a'];
  if (st == -1) {
    st = inf;
    int a, b;
    for (a = l; a < r; a = b) {
      b = a;
      while (b < r and s[b] == s[a]) ++b;
      st = min(st, solve(l, a, s[a]) + solve(b, r, s[a]) + (s[a] != c));
    }
  }
  return st;
}
int main() {
  memset(f, -1, sizeof f);
  ios::sync_with_stdio(0);
  cin.tie(0);
  int n;
  cin >> n;
  cin >> s;
  cout << solve(0, n, 'z' + 1) << endl;
}
