function Main(input) {
  var output = "";

  switch (input) {
    case "A":
      output = "T";
      break;
    case "T":
      output = "A";
      break;
    case "C":
      outeput = "G";
    case "G":
      output = "C";
  }

  console.log("%s", output);
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));
