Nokia keypad

def letter_combinations(digits):
    if not digits or any(d in '01' for d in digits):
        return "No Combination of Strings"
    
    phone = {
        '2': 'abc',
        '3': 'def',
        '4': 'ghi',
        '5': 'jkl',
        '6': 'mno',
        '7': 'pqrs',
        '8': 'tuv',
        '9': 'wxyz'
    }
    
    result = []
    
    def backtrack(index, path):
        if index == len(digits):
            result.append(path)
            return
        
        digit = digits[index]
        for letter in phone[digit]:
            backtrack(index + 1, path + letter)
    
    backtrack(0, "")
    return " ".join(result)

digits = input().strip()
print(letter_combinations(digits))