import java.util.*;
public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String s = sc.next();
    int n = s.length();
    int count = 0;
    int ans = 0;
    for (int i = 0; i < n; i++) {
      if (s.charAt(i) == 'A' || s.charAt(i) == 'C' || s.charAt(i) == 'G' || s.charAt(i) == 'T') {
        count++;
      } else {
        ans = Math.max(ans, count);
        count = 0;
      }
    }
    ans = Math.max(ans, count);
    System.out.println(ans);
  }
} import java.util.*; util.* java public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String s = sc.next();
    int n = s.length();
    int count = 0;
    int ans = 0;
    for (int i = 0; i < n; i++) {
      if (s.charAt(i) == 'A' || s.charAt(i) == 'C' || s.charAt(i) == 'G' || s.charAt(i) == 'T') {
        count++;
      } else {
        ans = Math.max(ans, count);
        count = 0;
      }
    }
    ans = Math.max(ans, count);
    System.out.println(ans);
  }
} Main public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String s = sc.next();
    int n = s.length();
    int count = 0;
    int ans = 0;
    for (int i = 0; i < n; i++) {
      if (s.charAt(i) == 'A' || s.charAt(i) == 'C' || s.charAt(i) == 'G' || s.charAt(i) == 'T') {
        count++;
      } else {
        ans = Math.max(ans, count);
        count = 0;
      }
    }
    ans = Math.max(ans, count);
    System.out.println(ans);
  } main {
    Scanner sc = new Scanner(System.in);
    String s = sc.next();
    int n = s.length();
    int count = 0;
    int ans = 0;
    for (int i = 0; i < n; i++) {
      if (s.charAt(i) == 'A' || s.charAt(i) == 'C' || s.charAt(i) == 'G' || s.charAt(i) == 'T') {
        count++;
      } else {
        ans = Math.max(ans, count);
        count = 0;
      }
    }
    ans = Math.max(ans, count);
    System.out.println(ans);
  } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) String s = sc.next(); s sc.next() sc.next sc int n = s.length(); n s.length() s.length s int count = 0; count 0 int ans = 0; ans 0 for (int i = 0; i < n; i++) {
      if (s.charAt(i) == 'A' || s.charAt(i) == 'C' || s.charAt(i) == 'G' || s.charAt(i) == 'T') {
        count++;
      } else {
        ans = Math.max(ans, count);
        count = 0;
      }
    } int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
      if (s.charAt(i) == 'A' || s.charAt(i) == 'C' || s.charAt(i) == 'G' || s.charAt(i) == 'T') {
        count++;
      } else {
        ans = Math.max(ans, count);
        count = 0;
      }
    } {
      if (s.charAt(i) == 'A' || s.charAt(i) == 'C' || s.charAt(i) == 'G' || s.charAt(i) == 'T') {
        count++;
      } else {
        ans = Math.max(ans, count);
        count = 0;
      }
    } if (s.charAt(i) == 'A' || s.charAt(i) == 'C' || s.charAt(i) == 'G' || s.charAt(i) == 'T') {
        count++;
      } else {
        ans = Math.max(ans, count);
        count = 0;
      } s.charAt(i) == 'A' || s.charAt(i) == 'C' || s.charAt(i) == 'G' || s.charAt(i) == 'T' s.charAt(i) == 'G' s.charAt(i) == 'A' || s.charAt(i) == 'C' || s.charAt(i) == 'G' || s.charAt(i) == 'T' s.charAt(i) == 'A' s.charAt(i) s.charAt s i 'A' s.charAt(i) == 'C' s.charAt(i) s.charAt s i 'C' s.charAt(i) == 'G' s.charAt(i) s.charAt s i 'G' s.charAt(i) == 'T' s.charAt(i) s.charAt s i 'T' {
        count++;
      } count++ count {
        ans = Math.max(ans, count);
        count = 0;
      } ans = Math.max(ans, count) ans Math.max(ans, count) Math.max Math ans count count = 0 count 0 ans = Math.max(ans, count) ans Math.max(ans, count) Math.max Math ans count System.out.println(ans) System.out.println System.out System System.out ans String[] args args public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String s = sc.next();
    int n = s.length();
    int count = 0;
    int ans = 0;
    for (int i = 0; i < n; i++) {
      if (s.charAt(i) == 'A' || s.charAt(i) == 'C' || s.charAt(i) == 'G' || s.charAt(i) == 'T') {
        count++;
      } else {
        ans = Math.max(ans, count);
        count = 0;
      }
    }
    ans = Math.max(ans, count);
    System.out.println(ans);
  }
} public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String s = sc.next();
    int n = s.length();
    int count = 0;
    int ans = 0;
    for (int i = 0; i < n; i++) {
      if (s.charAt(i) == 'A' || s.charAt(i) == 'C' || s.charAt(i) == 'G' || s.charAt(i) == 'T') {
        count++;
      } else {
        ans = Math.max(ans, count);
        count = 0;
      }
    }
    ans = Math.max(ans, count);
    System.out.println(ans);
  }
} Main
import java.util.*;
public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String s = sc.next();
    int n = s.length();
    int count = 0;
    int ans = 0;
    for (int i = 0; i < n; i++) {
      if (s.charAt(i) == 'A' || s.charAt(i) == 'C' || s.charAt(i) == 'G' || s.charAt(i) == 'T') {
        count++;
      } else {
        ans = Math.max(ans, count);
        count = 0;
      }
    }
    ans = Math.max(ans, count);
    System.out.println(ans);
  }
}
