function Main (input) {
  switch (input) {
    case 'A':
      console.log('T');
      break;
    case 'C':
      console.log('G');
      break;
    case 'G':
      console.log('C');
      break;
    case 'T':
      console.log('A');
      break;
  }
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));