def solution(S):
    import random
    import string
    length_of_s = len(S)
 
    S = list(S)
    
    for i in range(length_of_s//2):
     
        if S[i] == "?" and S[length_of_s - i - 1] != "?":
            S[i] = S[length_of_s - i - 1]
      
        elif S[i] == "?" and S[length_of_s - i - 1] == "?":
            random_letter = random.choice(string.ascii_letters).lower()
            S[length_of_s - i - 1] = random_letter
            S[i] = random_letter
            
        elif S[i] != "?" and S[length_of_s - i - 1] == "?":
            S[length_of_s - i - 1] = S[i]
       
        elif S[i] ==  S[length_of_s - i - 1]:
            continue

        else:
            if S[i] != S[length_of_s - i - 1]:
                return "NO"
    
    # List to string
    polindrome_string = "".join(S)
    return polindrome_string