def swap(n):
	if n == 'o':
		return '*'
	else:
		return 'o'
def fan_ying_bi(a,b):
	s = 0
	for i in range(len(a)):
		if a[i] != b[i]:
			a[i] = swap(a[i])
			a[i+1] = swap(a[i+1])
			s += 1
	return s
		
def main():
    a = list(input())
    b = list(input())
    print(fan_ying_bi(a,b))
    
if __name__ == '__main__':
    main()