def min_replacements(original, current):
    if len(original) != len(current):
        raise ValueError("Blueprint and current ship must have the same length")

    replacements = 0
    for o, c in zip(original, current):
        if o != c:
            replacements += 1

    return replacements


# Example usage
original = input().strip()   # blueprint string
current = input().strip()    # current ship string

print(min_replacements(original, current))