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

typedef long long ll;

int main() {
	char c;
	cin >> c;
	if (c == 'A')
		cout << 'T' << endl;
	else if (c == 'C')
		cout << 'G' << endl;
	else if (c == 'G')
		cout << 'C' << endl;
	else if (c == 'T')
		cout << 'A' << endl;
	return 0;
}