func readInts() -> [Int] {
    return readLine()!.split(separator: " ").map { Int($0)! }
}

func main() {
    let s = Array(readLine()!)
    let t = Array(readLine()!)
    
    var ans = 0
    for i in 0..<s.count {
        if s[i] != t[i] {
            ans += 1
        }
    }
    
    print(ans)
}

main()
