#include <iostream>
#include <vector>
#include <string>
#include <cmath>
#include <algorithm>
#include <functional>
#include <list>
#include <map>
#include <set>
using namespace std;

char str[5001];
short DP[2][5001],len,i,j;

int main()
{     

    scanf("%d\n%s",&len,str+1);
        
    for(i = 1;i <= len;++i)
      for(j = 1;j <= len;++j)
      {
        if(str[i] == str[len-j+1])
          DP[i%2][j] = DP[(i-1)%2][j-1] + 1;
        else
          DP[i%2][j] = DP[(i-1)%2][j]>DP[i%2][j-1] ? DP[(i-1)%2][j] : DP[i%2][j-1];      
      }
    printf("%d\n",len-DP[len%2][len]);
  

    system("pause");
    return 0;
};