import java.io.; import java.math.; import java.security.; import java.text.; import java.util.; import java.util.concurrent.; import java.util.function.; import java.util.regex.; import java.util.stream.*; import static java.util.stream.Collectors.joining; import static java.util.stream.Collectors.toList;

class Result {

public static int steadyGene(String gene) { 
    int a =0, c=0, g=0, t=0;
    for(int i=0; i<gene.length(); i++)
    {
        if(gene.charAt(i)=='A')a+=1;
        else if(gene.charAt(i)=='C')c+=1;
        else if(gene.charAt(i)=='G')g+=1;
        else t+=1;
    }
    int n = gene.length()/4;
    int A =0, C = 0, G = 0, T = 0, tot = 0;
    if(a>n)
    {
        A = a-n;tot+=A;
    }
    if(c>n)
    {
        C = c-n;tot+=C;
    }
    if(g>n)
    {
        G = g-n;tot+=G;
    }
    if(t>n)
    {
        T = t-n;tot+=T;
    } 
    while(true)
    {
        String s= gene.substring(0, tot+1);
        a=0;c=0;g=0;t=0;
        for(int i=0; i<tot; i++)
        {
            if(s.charAt(i)=='A')a+=1;
            else if(s.charAt(i)=='C')c+=1;
            else if(s.charAt(i)=='G')g+=1;
            else t+=1;
        }
        for(int i=0; i<gene.length()-tot; i++)
        {
            if(i>0)
            { 
                if(gene.charAt(i-1)=='A')a-=1;
                else if(gene.charAt(i-1)=='C')c-=1;
                else if(gene.charAt(i-1)=='G')g-=1;
                else t-=1;

                if(gene.charAt(i+tot-1)=='A')a+=1;
                else if(gene.charAt(i+tot-1)=='C')c+=1;
                else if(gene.charAt(i+tot-1)=='G')g+=1;
                else t+=1;
            }
            if(a>=A && c>=C && g>=G && t>=T)
            {
                return tot;
            }
        }
        tot++;
        if(tot>gene.length())
        break;
    }
    return 0;   
}

}

public class Solution { public static void main(String[] args) throws IOException { BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(System.in)); BufferedWriter bufferedWriter = new BufferedWriter(new FileWriter(System.getenv("OUTPUT_PATH")));

    int n = Integer.parseInt(bufferedReader.readLine().trim());

    String gene = bufferedReader.readLine();

    int result = Result.steadyGene(gene);

    bufferedWriter.write(String.valueOf(result));
    bufferedWriter.newLine();

    bufferedReader.close();
    bufferedWriter.close();
}

}