Untitled

import sys

n = int(input())
str = input()
x = input()

freq = {}
for char in str:
    if char in freq:
        freq[char] += 1 
    else:
        freq[char] = 1
        
max_freq = 0 
max_char = ''
for char, f in freq.items():
    if f > max_freq:
        max_freq = f
        max_char = char
    elif f == max_freq and ord(char) < ord(max_char):
        max_char = char
        
result = ''        
for char in str:
    if char == max_char:
        result += x
    else:
        result += char
        
print(result if len(result) > 0 else "NULL")