def letter_Combination(digits):
        keypad={"2":"ABC","3":"DEF","4":"GHI","5":"JKL","6":"MNO","7":"PQRS","8":"TUV","9":"WXYZ"
        }
        if backtrack(index,current):
            if index==len(digits):
                result.append(current)
            return[]
                      
                                          for ch in keypad[digits[index]]:

                        backtrack(index+1,current+ch)
                        backtrack(0,"")
                        return result
                        digits = input().strinp()
                        if not digits.isdigits():
                            print("Invalid input")
                            else:
                                for d in digits:
                                    if d< '2' or d> '9':
                                        print("Invalid input")
                                        exit()
                                        if digits=="":
                                            else:
                                                output = letter_combinations(digits)
                                                print("[" + " ,".join(output)+ "]")