function Main(input) {

	switch(input) {
		  case "A":
      	console.log('T');
        break;
			case "T":
      	console.log('A');
        break;
			case "G":
      	console.log('C');
        break;
			case "C":
      	console.log('G');
        break;

	}
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));