using System;
using System.Text;
using System.Linq;
using System.Collections;
using System.Collections.Generic;
using static System.Console;
using static System.Math;

namespace AtCoder
{
    public class Program
    {
        public static void Main(string[] args)
        {
            new Program().Solve(new ConsoleInput(Console.In, ' '));
        }

        public void Solve(ConsoleInput cin)
        {
            var S = cin.Read;

            var start = -1;
            var end = -1;
            var ans = 0;
            for (var i = 0; i < S.Length; i++)
            {
                if (S[i] == 'A' || S[i] == 'C' || S[i] == 'G' || S[i] == 'T')
                {
                    if (start == -1)
                    {
                        start = i;
                    }
                }
                else
                {
                    if (start != -1)
                    {
                        end = i;
                        ans = Max(ans, end - start);
                        start = -1;
                    }
                    continue;
                }
            }
            if (start != -1)
            {
                end = S.Length;
                ans = Max(ans, end - start);
                start = -1;
            }
            WriteLine(ans);

        }

        static long GcdArray(long[] array)
        {
            var ans = array[0];
            for(var i=1; i<array.Length; i++)
            {
                ans = Gcd(ans, array[i]);
            }
            return ans;
        }

        static long Gcd(long a, long b)
        {
            var v = new[] { a, b };
            while (v[1] != 0) { v = new[] { v[1], v[0] % v[1] }; }
            return v[0];
        }

        static long Lcm(long a, long b)
        {
            return a / Gcd(a, b) * b;
        }
    }

    public class ConsoleInput
    {
        private readonly System.IO.TextReader _stream;
        private char _separator = ' ';
        private Queue<string> inputStream;
        public ConsoleInput(System.IO.TextReader stream, char separator = ' ')
        {
            this._separator = separator;
            this._stream = stream;
            inputStream = new Queue<string>();
        }
        public string Read
        {
            get
            {
                if (inputStream.Count != 0) return inputStream.Dequeue();
                string[] tmp = _stream.ReadLine().Split(_separator);
                for (int i = 0; i < tmp.Length; ++i)
                    inputStream.Enqueue(tmp[i]);
                return inputStream.Dequeue();
            }
        }
        public string ReadLine { get { return _stream.ReadLine(); } }
        public int ReadInt { get { return int.Parse(Read); } }
        public long ReadLong { get { return long.Parse(Read); } }
        public double ReadDouble { get { return double.Parse(Read); } }
        public string[] ReadStrArray(long N) { var ret = new string[N]; for (long i = 0; i < N; ++i) ret[i] = Read; return ret; }
        public int[] ReadIntArray(long N) { var ret = new int[N]; for (long i = 0; i < N; ++i) ret[i] = ReadInt; return ret; }
        public long[] ReadLongArray(long N) { var ret = new long[N]; for (long i = 0; i < N; ++i) ret[i] = ReadLong; return ret; }
    }
}

Measured memory_bytes: 11220