#!/usr/bin/env python
"""
Counting Point Mutations
"""

def main():
    with open('src')  as src, open('dst', 'w+') as dst:
        first = src.readline().strip()
        second = src.readline().strip()
        diff = sum(int(f != s) for f, s in zip(first, second))
        dst.write(str(diff))

if __name__ == '__main__':
    main()