#include <bits/stdc++.h>

using namespace std;

int main()
{
   int n; cin >> n;
   string s; cin >> s;
   
   /*
      dp[i][j] says if the substring from i to j is a palindrome
   */
   vector<vector<int>> dp(n, vector<int>(n, 0));
   for (int i = n - 1; i >= 0; i--)
   {
      dp[i][i] = 1;
      for (int j = i + 1; j < n; j++)
      {
         if (s[i] == s[j])
         {
            dp[i][j] = dp[i + 1][j - 1] + 2;
         }
         else
         {
            dp[i][j] = max(dp[i + 1][j], dp[i][j - 1]);
         }
      }
   }

   cout <<  dp[0][n - 1] << endl;

   return 0;
}