import java.util.Scanner

object Main {
  def main(args: Array[String]) {
    val result = solve(new Scanner(System.in))
    if (result.length > 0) println(result)
  }

  def solve(in: String): String = solve(new Scanner(in))

  def solve(sc: Scanner): String = {
    val b = sc.next

    if (b == "A") "T"
    else if (b == "C") "G"
    else if (b == "G") "C" else "A"
  }
}
