using System;
using System.Collections;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Numerics;
using System.Runtime.InteropServices;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading.Tasks;
using static System.Math;
using static Math2;
using static io;
using static Utils;

public class CP {
  public void Solve() {
    var S = Str;
    int ma = 0;
    int cnt = 0;
    for (int i = 0; i < S.Length; ++i) {
      if (S[i] == 'A' || S[i] == 'T' || S[i] == 'G' || S[i] == 'C') ++cnt;
      else {
        ma = Max(ma, cnt);
        cnt = 0;

      }
    }
    ma = Max(ma, cnt);
    Put(ma);
  }
}
public static class NumberTheory {
  public static Dictionary<long, int> PrimeFactorization(long N) {
    var ret = new Dictionary<long, int>();
    for (long i = 2; i * i <= N; ++i) {
      int cnt = 0;
      while (N % i == 0) {
        cnt++;
        N /= i;
      }
      if (cnt != 0) ret[i] = cnt;
    }
    if (N >= 2) ret[N] = 1;
    return ret;
  }
  public static Dictionary<int, int> FactoricalPF(int N) {
    var ret = new Dictionary<int, int>();
    for (int i = 2; i <= N; ++i) {
      int ni = i;
      for (int j = 2; j * j <= ni; ++j) {
        int cnt = 0;
        while (ni % j == 0) {
          ++cnt;
          ni /= j;
        }
        if (cnt == 0) continue;
        if (ret.ContainsKey(j))
          ret[j] += cnt;
        else ret[j] = cnt;
      }
      if (ni >= 2) {
        if (ret.ContainsKey(ni))
          ret[ni] += 1;
        else ret[ni] = 1;
      }
    }
    return ret;
  }
  public static List<long> DivisorEnumrate(long N) {
    var ret = new List<long>();
    for (long i = 1; i * i <= N; ++i) {
      if (N % i == 0) {
        ret.Add(i);
        ret.Add(N / i);
      }
    }
    return ret;
  }
  public static List<int> seive(int n) {
    var ret = new List<int>();
    bool[] isPrime = new bool[n + 1];
    for (int i = 1; i <= n; ++i) isPrime[i] = true;
    isPrime[1] = false;
    for (int i = 1; i <= n; ++i) {
      int j = i;
      if (isPrime[i]) ret.Add(i);
      else continue;
      while (j <= n) {
        j += i;
        if (j > n) continue;
        isPrime[j] = false;
      }
    }
    return ret;
  }
  public static bool isPrime(long n) {
    for (int i = 2; i * i <= n; ++i) {
      if (n % i == 0) return false;
    }
    return true;
  }
}

public class BinarySearch {
  public static int Lower_Bound<T>(IList<T> Array, T Target) {
    return Lower_Bound<T>(Array, Target, Comparer<T>.Default.Compare);
  }
  public static int Lower_Bound<T>(IList<T> Array, T Target, Comparison<T> Comp) {
    var cmp = Comparer<T>.Create(Comp);
    var l = -1; //always ng
    var r = Array.Count(); //always ok
    while (r - l > 1) {
      var mid = l + (r - l) / 2;
      var res = cmp.Compare(Array[mid], Target);
      if (res >= 0) r = mid;
      else l = mid;
    }
    return r;
  }
  public static int Upper_Bound<T>(IList<T> Array, T Target) {
    return Upper_Bound<T>(Array, Target, Comparer<T>.Default.Compare);
  }
  public static int Upper_Bound<T>(IList<T> Array, T Target, Comparison<T> Comp) {
    var cmp = Comparer<T>.Create(Comp);
    var l = -1;
    var r = Array.Count;
    while (r - l > 1) {
      var mid = l + (r - l) / 2;
      var res = cmp.Compare(Array[mid], Target);
      if (res > 0) r = mid;
      else l = mid;
    }
    return r;
  }
}
public static class Math2 {
  public const int INF = 1 << 29;
  public const long INFL = 1L << 60;
  public const long MOD = 1000000007;
  public const int MOD2 = 998244353;

  public static long Power(long a, long b, long MOD = 1000000007) //i^N
  {
    long res = 1;
    while (b > 0) {
      if ((b & 1) != 0) res = res * a % MOD;
      a = a * a % MOD;
      b >>= 1;
    }
    return res;
  }
  public static long Power2(long a, long b) //i^N
  {
    long res = 1;
    while (b > 0) {
      if ((b & 1) != 0) res = res * a;
      a = a * a;
      b >>= 1;
    }
    return res;
  }
  public static long GCD(long a, long b) {
    while (b > 0) {
      var r = a % b;
      a = b;
      b = r;
    }
    return a;
  }
  public static long LCM(long a, long b) => (b / GCD(a, b)) * a;
  public static long Comb(long n, long r, int MOD = 1000000007) {
    if (r > n - r) r = n - r;
    long Nume = 1;
    long Deno = 1;

    if (r > n - r) r = n - r;
    for (long i = 1; i <= r; ++i) {
      Deno = (Deno * i) % MOD;
      Nume = Nume * (n - i + 1) % MOD;
    }
    return (Nume * inv(Deno)) % MOD;
  }
  public static long Comb2(long n, long r) {
    long Nume = 1;
    long Deno = 1;

    if (r > n - r) r = n - r;
    for (long i = 1; i <= r; ++i) {
      Deno *= i;
      Nume *= n - i + 1;
    }
    return Nume / Deno;
  }
  public static long inv(long x, int MOD = 1000000007) {
    return Power(x, MOD - 2, MOD);
  }
}
public static class Utils {
  public static void Swap<T>(ref T A, ref T B) {
    T x = A;
    A = B;
    B = x;
  }
  public static int DigitSum(int N) {
    string s = N.ToString();
    int ret = 0;
    for (int i = 0; i < s.Length; ++i) ret += s[i] - '0';
    return ret;
  }
  public static string ConvertBase(long N, int K) {
    StringBuilder ret = new StringBuilder();
    while (N > 0) {
      var r = N % K;
      N /= K;
      ret.Append(r);
    }
    return new string(ret.ToString().ToCharArray().Reverse().ToArray());
  }
}
public class PriorityQueue<T> {
  List<T> _item;
  public int Count { get { return _item.Count; } }
  bool _isascend { get; set; }
  public T Peek { get { return _item[0]; } }
  Comparison<T> Comp;
  public PriorityQueue(bool IsAscend = true, IEnumerable<T> list = null) : this(Comparer<T>.Default.Compare, IsAscend, list) { }
  public PriorityQueue(Comparison<T> cmp, bool IsAscend = true, IEnumerable<T> list = null) {
    _item = new List<T>();
    _isascend = IsAscend;
    this.Comp = cmp;
    if (list != null) {
      _item.AddRange(list);
      Build();
    }
  }

  private int Compare(int i, int j) => (_isascend ? -1 : 1) * Comp(_item[i], _item[j]);
  private void Swap(int i, int j) { var t = _item[i]; _item[i] = _item[j]; _item[j] = t; }
  private int Parent(int i) => (i - 1) >> 1;
  private int Left(int i) => (i << 1) + 1;
  public T Enqueue(T val) {
    int i = _item.Count;
    _item.Add(val);
    while (i > 0) {
      int p = Parent(i);
      if (Compare(i, p) > 0)
        Swap(i, p);
      i = p;
    }
    return val;
  }
  private void Heapify(int index) {
    for (int i = index, j;
      (j = Left(i)) < _item.Count; i = j) {
      if (j != _item.Count - 1 && Compare(j, j + 1) < 0) j++;
      if (Compare(i, j) < 0)
        Swap(i, j);
    }
  }
  public T Dequeue() {
    T val = _item[0];
    _item[0] = _item[_item.Count - 1];
    _item.RemoveAt(_item.Count - 1);
    Heapify(0);
    return val;
  }
  private void Build() {
    for (var i = (_item.Count >> 1) - 1; i >= 0; i--)
      Heapify(i);
  }
}
class Program {
  static void Main(string[] args) {
    var CP = new CP();
    CP.Solve();
  }
}
public static class io {
  public static string Str => Console.ReadLine();
  public static string[] Strs => Str.Split(' ');
  public static long[] Longs => Strs.Select(long.Parse).ToArray();
  public static int[] Ints => Strs.Select(int.Parse).ToArray();
  public static char[] Chars => Str.ToArray();
  public static double[] Doubles => Strs.Select(double.Parse).ToArray();
  public static long Long1 => Longs[0];
  public static int Int1 => Ints[0];
  public static char Char1 => Chars[0];
  public static double Double1 => Doubles[0];
  public static long[] VerticalRead(int N) { long[] A = new long[N]; for (int i = 0; i < N; ++i) A[i] = Long1; return A; }
  public static void Put(string a) => Console.WriteLine(a);
  public static void Put(params object[] i) => Put(string.Join(" ", i));
  public static void Put<T>(IEnumerable<T> a) => Put(string.Join(" ", a));
  public static void VerticalPut<T>(IEnumerable<T> a) { foreach (var z in a) Put(z); }
  public static void YN(bool i) {
    if (i) Put("Yes");
    else Put("No");
  }
}

Measured memory_bytes: 13140