import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

import java.util.StringTokenizer;

public class Main {
	public static void main(String[] args) {
		FastReader sc = new FastReader();
		String s = sc.next();
		
		int begin = 0;
		int end = 0;
		int res = 0;
		
		while (end < s.length()) {
			char c = s.charAt(end);
			if (c == 'A' || c == 'C' || c == 'G' || c == 'T') {
				res = Math.max(res, end - begin + 1);
				end++;
			} else {
				end++;
				begin = end;
			}
		}
		System.out.println(res);
	}
	
	static class FastReader { 
        BufferedReader br; 
        StringTokenizer st; 
  
        public FastReader() { 
            br = new BufferedReader(new InputStreamReader(System.in)); 
        } 
  
        String next() { 
            while (st == null || !st.hasMoreElements()) { 
                try { 
                    st = new StringTokenizer(br.readLine()); 
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } 
  
        int nextInt() { 
            return Integer.parseInt(next()); 
        } 
  
        long nextLong() { 
            return Long.parseLong(next()); 
        } 
  
        double nextDouble() { 
            return Double.parseDouble(next()); 
        } 
  
        String nextLine() { 
            String str = ""; 
            try{ 
                str = br.readLine(); 
            } catch (IOException e) { 
                e.printStackTrace(); 
            } 
            return str; 
        } 
    } 
} import java.io.BufferedReader; BufferedReader java.io import java.io.IOException; IOException java.io import java.io.InputStreamReader; InputStreamReader java.io import java.util.StringTokenizer; StringTokenizer java.util public class Main {
	public static void main(String[] args) {
		FastReader sc = new FastReader();
		String s = sc.next();
		
		int begin = 0;
		int end = 0;
		int res = 0;
		
		while (end < s.length()) {
			char c = s.charAt(end);
			if (c == 'A' || c == 'C' || c == 'G' || c == 'T') {
				res = Math.max(res, end - begin + 1);
				end++;
			} else {
				end++;
				begin = end;
			}
		}
		System.out.println(res);
	}
	
	static class FastReader { 
        BufferedReader br; 
        StringTokenizer st; 
  
        public FastReader() { 
            br = new BufferedReader(new InputStreamReader(System.in)); 
        } 
  
        String next() { 
            while (st == null || !st.hasMoreElements()) { 
                try { 
                    st = new StringTokenizer(br.readLine()); 
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } 
  
        int nextInt() { 
            return Integer.parseInt(next()); 
        } 
  
        long nextLong() { 
            return Long.parseLong(next()); 
        } 
  
        double nextDouble() { 
            return Double.parseDouble(next()); 
        } 
  
        String nextLine() { 
            String str = ""; 
            try{ 
                str = br.readLine(); 
            } catch (IOException e) { 
                e.printStackTrace(); 
            } 
            return str; 
        } 
    } 
} Main public static void main(String[] args) {
		FastReader sc = new FastReader();
		String s = sc.next();
		
		int begin = 0;
		int end = 0;
		int res = 0;
		
		while (end < s.length()) {
			char c = s.charAt(end);
			if (c == 'A' || c == 'C' || c == 'G' || c == 'T') {
				res = Math.max(res, end - begin + 1);
				end++;
			} else {
				end++;
				begin = end;
			}
		}
		System.out.println(res);
	} main {
		FastReader sc = new FastReader();
		String s = sc.next();
		
		int begin = 0;
		int end = 0;
		int res = 0;
		
		while (end < s.length()) {
			char c = s.charAt(end);
			if (c == 'A' || c == 'C' || c == 'G' || c == 'T') {
				res = Math.max(res, end - begin + 1);
				end++;
			} else {
				end++;
				begin = end;
			}
		}
		System.out.println(res);
	} FastReader sc = new FastReader(); sc new FastReader() String s = sc.next(); s sc.next() sc.next sc int begin = 0; begin 0 int end = 0; end 0 int res = 0; res 0 while (end < s.length()) {
			char c = s.charAt(end);
			if (c == 'A' || c == 'C' || c == 'G' || c == 'T') {
				res = Math.max(res, end - begin + 1);
				end++;
			} else {
				end++;
				begin = end;
			}
		} end < s.length() end s.length() s.length s {
			char c = s.charAt(end);
			if (c == 'A' || c == 'C' || c == 'G' || c == 'T') {
				res = Math.max(res, end - begin + 1);
				end++;
			} else {
				end++;
				begin = end;
			}
		} char c = s.charAt(end); c s.charAt(end) s.charAt s end if (c == 'A' || c == 'C' || c == 'G' || c == 'T') {
				res = Math.max(res, end - begin + 1);
				end++;
			} else {
				end++;
				begin = end;
			} c == 'A' || c == 'C' || c == 'G' || c == 'T' c == 'G' c == 'A' || c == 'C' || c == 'G' || c == 'T' c == 'A' c 'A' c == 'C' c 'C' c == 'G' c 'G' c == 'T' c 'T' {
				res = Math.max(res, end - begin + 1);
				end++;
			} res = Math.max(res, end - begin + 1) res Math.max(res, end - begin + 1) Math.max Math res end - begin + 1 end - begin end begin 1 end++ end {
				end++;
				begin = end;
			} end++ end begin = end begin end System.out.println(res) System.out.println System.out System System.out res String[] args args static class FastReader { 
        BufferedReader br; 
        StringTokenizer st; 
  
        public FastReader() { 
            br = new BufferedReader(new InputStreamReader(System.in)); 
        } 
  
        String next() { 
            while (st == null || !st.hasMoreElements()) { 
                try { 
                    st = new StringTokenizer(br.readLine()); 
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } 
  
        int nextInt() { 
            return Integer.parseInt(next()); 
        } 
  
        long nextLong() { 
            return Long.parseLong(next()); 
        } 
  
        double nextDouble() { 
            return Double.parseDouble(next()); 
        } 
  
        String nextLine() { 
            String str = ""; 
            try{ 
                str = br.readLine(); 
            } catch (IOException e) { 
                e.printStackTrace(); 
            } 
            return str; 
        } 
    } FastReader BufferedReader br; br StringTokenizer st; st public FastReader() { 
            br = new BufferedReader(new InputStreamReader(System.in)); 
        } FastReader { 
            br = new BufferedReader(new InputStreamReader(System.in)); 
        } br = new BufferedReader(new InputStreamReader(System.in)) br new BufferedReader(new InputStreamReader(System.in)) String next() { 
            while (st == null || !st.hasMoreElements()) { 
                try { 
                    st = new StringTokenizer(br.readLine()); 
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } next { 
            while (st == null || !st.hasMoreElements()) { 
                try { 
                    st = new StringTokenizer(br.readLine()); 
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } while (st == null || !st.hasMoreElements()) { 
                try { 
                    st = new StringTokenizer(br.readLine()); 
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } 
            } st == null || !st.hasMoreElements() st == null st null !st.hasMoreElements() st.hasMoreElements() st.hasMoreElements st { 
                try { 
                    st = new StringTokenizer(br.readLine()); 
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } 
            } try { 
                    st = new StringTokenizer(br.readLine()); 
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } IOException  e { 
                    e.printStackTrace(); 
                } e.printStackTrace() e.printStackTrace e { 
                    st = new StringTokenizer(br.readLine()); 
                } st = new StringTokenizer(br.readLine()) st new StringTokenizer(br.readLine()) return st.nextToken(); st.nextToken() st.nextToken st int nextInt() { 
            return Integer.parseInt(next()); 
        } nextInt { 
            return Integer.parseInt(next()); 
        } return Integer.parseInt(next()); Integer.parseInt(next()) Integer.parseInt Integer next() next long nextLong() { 
            return Long.parseLong(next()); 
        } nextLong { 
            return Long.parseLong(next()); 
        } return Long.parseLong(next()); Long.parseLong(next()) Long.parseLong Long next() next double nextDouble() { 
            return Double.parseDouble(next()); 
        } nextDouble { 
            return Double.parseDouble(next()); 
        } return Double.parseDouble(next()); Double.parseDouble(next()) Double.parseDouble Double next() next String nextLine() { 
            String str = ""; 
            try{ 
                str = br.readLine(); 
            } catch (IOException e) { 
                e.printStackTrace(); 
            } 
            return str; 
        } nextLine { 
            String str = ""; 
            try{ 
                str = br.readLine(); 
            } catch (IOException e) { 
                e.printStackTrace(); 
            } 
            return str; 
        } String str = ""; str "" try{ 
                str = br.readLine(); 
            } catch (IOException e) { 
                e.printStackTrace(); 
            } catch (IOException e) { 
                e.printStackTrace(); 
            } IOException e { 
                e.printStackTrace(); 
            } e.printStackTrace() e.printStackTrace e { 
                str = br.readLine(); 
            } str = br.readLine() str br.readLine() br.readLine br return str; str public class Main {
	public static void main(String[] args) {
		FastReader sc = new FastReader();
		String s = sc.next();
		
		int begin = 0;
		int end = 0;
		int res = 0;
		
		while (end < s.length()) {
			char c = s.charAt(end);
			if (c == 'A' || c == 'C' || c == 'G' || c == 'T') {
				res = Math.max(res, end - begin + 1);
				end++;
			} else {
				end++;
				begin = end;
			}
		}
		System.out.println(res);
	}
	
	static class FastReader { 
        BufferedReader br; 
        StringTokenizer st; 
  
        public FastReader() { 
            br = new BufferedReader(new InputStreamReader(System.in)); 
        } 
  
        String next() { 
            while (st == null || !st.hasMoreElements()) { 
                try { 
                    st = new StringTokenizer(br.readLine()); 
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } 
  
        int nextInt() { 
            return Integer.parseInt(next()); 
        } 
  
        long nextLong() { 
            return Long.parseLong(next()); 
        } 
  
        double nextDouble() { 
            return Double.parseDouble(next()); 
        } 
  
        String nextLine() { 
            String str = ""; 
            try{ 
                str = br.readLine(); 
            } catch (IOException e) { 
                e.printStackTrace(); 
            } 
            return str; 
        } 
    } 
} public class Main {
	public static void main(String[] args) {
		FastReader sc = new FastReader();
		String s = sc.next();
		
		int begin = 0;
		int end = 0;
		int res = 0;
		
		while (end < s.length()) {
			char c = s.charAt(end);
			if (c == 'A' || c == 'C' || c == 'G' || c == 'T') {
				res = Math.max(res, end - begin + 1);
				end++;
			} else {
				end++;
				begin = end;
			}
		}
		System.out.println(res);
	}
	
	static class FastReader { 
        BufferedReader br; 
        StringTokenizer st; 
  
        public FastReader() { 
            br = new BufferedReader(new InputStreamReader(System.in)); 
        } 
  
        String next() { 
            while (st == null || !st.hasMoreElements()) { 
                try { 
                    st = new StringTokenizer(br.readLine()); 
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } 
  
        int nextInt() { 
            return Integer.parseInt(next()); 
        } 
  
        long nextLong() { 
            return Long.parseLong(next()); 
        } 
  
        double nextDouble() { 
            return Double.parseDouble(next()); 
        } 
  
        String nextLine() { 
            String str = ""; 
            try{ 
                str = br.readLine(); 
            } catch (IOException e) { 
                e.printStackTrace(); 
            } 
            return str; 
        } 
    } 
} Main
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

import java.util.StringTokenizer;

public class Main {
	public static void main(String[] args) {
		FastReader sc = new FastReader();
		String s = sc.next();
		
		int begin = 0;
		int end = 0;
		int res = 0;
		
		while (end < s.length()) {
			char c = s.charAt(end);
			if (c == 'A' || c == 'C' || c == 'G' || c == 'T') {
				res = Math.max(res, end - begin + 1);
				end++;
			} else {
				end++;
				begin = end;
			}
		}
		System.out.println(res);
	}
	
	static class FastReader { 
        BufferedReader br; 
        StringTokenizer st; 
  
        public FastReader() { 
            br = new BufferedReader(new InputStreamReader(System.in)); 
        } 
  
        String next() { 
            while (st == null || !st.hasMoreElements()) { 
                try { 
                    st = new StringTokenizer(br.readLine()); 
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } 
  
        int nextInt() { 
            return Integer.parseInt(next()); 
        } 
  
        long nextLong() { 
            return Long.parseLong(next()); 
        } 
  
        double nextDouble() { 
            return Double.parseDouble(next()); 
        } 
  
        String nextLine() { 
            String str = ""; 
            try{ 
                str = br.readLine(); 
            } catch (IOException e) { 
                e.printStackTrace(); 
            } 
            return str; 
        } 
    } 
}
