#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#define endl '\n'
using namespace std;

typedef long long ll;

string s;
int cnt, maxn = 0;

int main() {
	cin >> s;
	for (unsigned long long i = 0; i < s.length(); i++) {
		if (s[i] == 'A' || s[i] == 'C' || s[i] == 'G' || s[i] == 'T')
			cnt++;
		else {
			maxn = max(cnt, maxn);
			cnt = 0;
		}
	}
	maxn = max(cnt, maxn);	
	cout << maxn << endl;
	return 0;
}