using System;
using System.IO;
using System.Linq;
using System.Text;
using System.Diagnostics;
using System.Collections;
using System.Collections.Generic;

namespace atcoder
{
    class Solver
    {
        //Solve
        public void Solve()
        {
            cin = new Scanner();
            string s = cin.next;

            char[] t = s.ToCharArray();

            int max = 0;
            int z = 0;
            for (int i = 0; i < t.Length; i++)
            {
                if (t[i] == 'A' || t[i] == 'T' || t[i] == 'G' || t[i] == 'C')
                {
                    z++;
                    max = Math.Max(max, z);
                }
                else
                {
                    z = 0;
                }
            }

            Console.Write(max);
            return;
        }


        //Main
        public Solver() { }
        Scanner cin;
        public static int Main()
        {
            var sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
            Console.SetOut(sw);
            new Solver().Solve();
            Console.Out.Flush();
            return 0;
        }

        //Util
        static bool IsOdd(long num) { return (num & 1) == 1; }
        static bool IsEven(long num) { return (num & 1) == 0; }
        static long GCD(long a, long b) { return b != 0 ? GCD(b, a % b) : a; }
    }

    class Scanner
    {
        string[] s; int i;
        readonly char[] cs = new char[] { ' ' };
        public Scanner() { s = new string[0]; i = 0; }
        public string[] scan => Console.ReadLine().Split();
        public int[] scanint => Array.ConvertAll(scan, int.Parse);
        public long[] scanlong => Array.ConvertAll(scan, long.Parse);
        public double[] scandouble => Array.ConvertAll(scan, double.Parse);
        public string next
        {
            get
            {
                if (i < s.Length) return s[i++];
                string st = Console.ReadLine();
                while (st == "") st = Console.ReadLine();
                s = st.Split(cs, StringSplitOptions.RemoveEmptyEntries);
                i = 0;
                return next;
            }
        }
        public int nextint => int.Parse(next);
        public long nextlong => long.Parse(next);
        public double nextdouble => double.Parse(next);
    }

    class Node<T>
    {
        private int m_height;
        private Node<T> m_parent;
        public T Item { get; private set; }
        public Node(T item)
        {
            m_height = 0;
            m_parent = this;
            Item = item;
        }
        public Node<T> Find()
        {
            if (m_parent == this) { return this; }

            Node<T> parent = m_parent.Find();
            m_parent = parent;
            return parent;
        }
        public static void Unite(Node<T> a, Node<T> b)
        {
            Node<T> p = a.Find();
            Node<T> q = b.Find();
            if (p.m_height < q.m_height)
            {
                p.m_parent = q;
                q.m_height = Math.Max(p.m_height + 1, q.m_height);
            }
            else
            {
                q.m_parent = p;
                p.m_height = Math.Max(q.m_height + 1, p.m_height);
            }
        }
    }
}

Measured memory_bytes: 13140