def turn(a,k):
    if a[k] == '*':
        a[k] = 'o'
    else:
        a[k] = '*'

a = [c for c in input()]
b = input()

res = 0
for i in range(len(a)):
    if a[i] != b[i]:
        turn(a,i)
        turn(a, i + 1)
        res += 1
print(res)