import java.io.*;
import java.util.*;
import java.util.stream.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner();
        int n = sc.nextInt();
        char[] inputs = sc.next().toCharArray();
        BinaryIndexedTree bit = new BinaryIndexedTree(n + 1);
        for (int i = 1; i <= n; i++) {
            bit.add(i, 1);
        }
        ArrayList<Character> list = new ArrayList<>();
        int[] counts = new int[26];
        for (char c : inputs) {
            counts[c - 'A']++;
        }
        String agct = "AGCT";
        int[] idxes = new int[4];
        for (int i = 0; i < 4; i++) {
            idxes[i] = agct.charAt(i) - 'A';
        }
        int ans = 0;
        int plus = 0;
        while (true) {
            int current = 0;
            for (int x : idxes) {
                current += counts[(x + plus) % 26];
            }
            if (current == 0) {
                break;
            }
            int left = 0;
            int right = n;
            while (right - left > 1) {
                int m = (left + right) / 2;
                if (bit.getSum(m) < current) {
                    left = m;
                } else {
                    right = m;
                }
            }
            char c = inputs[right - 1];
            counts[c - 'A']--;
            bit.add(right, -1);
            plus += 26 - counts[c - 'A'] % 26;
            plus %= 26;
            ans++;
        }
        System.out.println(ans);
    }
}
class BinaryIndexedTree {
    int size;
    int[] tree;
    
    public BinaryIndexedTree(int size) {
        this.size = size;
        tree = new int[size];
    }
    
    public void add(int idx, int value) {
        int mask = 1;
        while (idx < size) {
            if ((idx & mask) != 0) {
                tree[idx] += value;
                idx += mask;
            }
            mask <<= 1;
        }
    }
    
    public int getSum(int from, int to) {
        return getSum(to) - getSum(from - 1);
    }
    
    public int getSum(int x) {
        int mask = 1;
        int ans = 0;
        while (x > 0) {
            if ((x & mask) != 0) {
                ans += tree[x];
                x -= mask;
            }
            mask <<= 1;
        }
        return ans;
    }
}
class Utilities {
    static String arrayToLineString(Object[] arr) {
        return Arrays.stream(arr).map(x -> x.toString()).collect(Collectors.joining("\n"));
    }
    
    static String arrayToLineString(int[] arr) {
        return String.join("\n", Arrays.stream(arr).mapToObj(String::valueOf).toArray(String[]::new));
    }
}
class Scanner {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    StringTokenizer st = new StringTokenizer("");
    StringBuilder sb = new StringBuilder();
    
    public Scanner() throws Exception {
        
    }
    
    public int nextInt() throws Exception {
        return Integer.parseInt(next());
    }
    
    public long nextLong() throws Exception {
        return Long.parseLong(next());
    }
    
    public double nextDouble() throws Exception {
        return Double.parseDouble(next());
    }
    
    public int[] nextIntArray() throws Exception {
        return Stream.of(br.readLine().split(" ")).mapToInt(Integer::parseInt).toArray();
    }
    
    public String next() throws Exception {
        while (!st.hasMoreTokens()) {
            st = new StringTokenizer(br.readLine());
        }
        return st.nextToken();
    }
    
}