import std.algorithm;
import std.array;
import std.conv;
import std.math;
import std.range;
import std.stdio;
import std.string;
import std.typecons;

T read(T)() { return readln.chomp.to!T; }
T[] reads(T)() { return readln.split.to!(T[]); }
alias readint = read!int;
alias readints = reads!int;

int calc(string s) {
    int ans = 0;
    int len = 0;
    foreach (c; s) {
        if ("ACGT".canFind(c)) len++;
        else len = 0;
        ans = max(ans, len);
    }
    return ans;
}

void main() {
    string s = read!string;
    writeln(calc(s));
}
