#include<cstdio>
#include<sstream>
#include<cstdlib>
#include<cctype>
#include<cmath>
#include<algorithm>
#include<set>
#include<queue>
#include<stack>
#include<list>
#include<iostream>
#include<fstream>
#include<numeric>
#include<string>
#include<vector>
#include<cstring>
#include<map>
#include<iterator>
using namespace std;
char s[5010];
int n,dp[5010][5010];

int count(int i,int n)
{
    if(dp[i][n]!=-1)
    return dp[i][n];
    if(i>=n)
    return 0;
    if(s[i]==s[n])
    return dp[i][n]=count(i+1,n-1);
    else
    return dp[i][n]=min(count (i+1,n)+1,count(i,n-1)+1);
}
int main()
{
memset(dp,-1,sizeof dp);

    cin >> n;
    getchar();
    gets(s);
    int i=count(0,n-1);
    cout << i << "\n" ;
    return 0;


}