str1 = input()
str2 = input()

# * for pos and o for neg

def getbool(ls):
    ans = []
    for _ in ls:
        if _ == "o":
            ans.append(False)
        else:
            ans.append(True)
    return ans

str1 = getbool(str1)
str2 = getbool(str2)

# defirn two indicators

cnt = 0
for i in range(len(str1)-1):
    if str1[i] != str2[i]:
        str1[i] = not(str1[i])
        str1[i+1] = not(str1[i+1])
        cnt += 1

print(cnt)