var str1: String = readLine()!
var str1List: [Character] = []

var str2: String = readLine()!
var str2List: [Character] = []
var mc = 0

for c in str1 {
    str1List.append(c)
}

for c in str2 {
    str2List.append(c)
}

for i in 0 ..< str1List.count {
    if (str1List[i] != str2List[i]) {
        mc += 1
    }
}

print(mc)