【競プロ精進ログ】ABC122-A

#include <bits/stdc++.h>
#define _GLIBCXX_DEBUG
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define repi(i, a, b) for (int i = (int)(a); i < (int)(b); i++)
using namespace std;
typedef long long ll;

char b;

int main(){
  cin >> b;
  switch (b){
    case 'A':
      cout << 'T' << endl;
      break;
    case 'T':
      cout << 'A' << endl;
      break;
    case 'G':
      cout << 'C' << endl;
      break;
    case 'C':
      cout << 'G' << endl;
      break;
  }
}