process.stdin.resume();
process.stdin.setEncoding('utf8');
 
var lines = [];
var reader = require('readline').createInterface({
	input: process.stdin,
	output: process.stdout
});
 
reader.on('line', (line) => {
	lines.push(line);
});
 
reader.on('close', () => {
  const S = lines[0];
  var ans = {A: "T", T:"A", G:"C", C:"G"}[S]
  console.log(ans);
});