func main() {
    let S = Array(readLine()!)
    let T = Array(readLine()!)
    
    //print((0..<S.count).compactMap { S.index[$0] == T.index[$0] ? 0 : 1 }.reduce(0) { $0 + $1 })
    print((1..<S.count).map { S[$0] == T[$0] ? 0 : 1 }.reduce(0, +))
}

mainfunc main() {
    func readInts() -> [Int] {
        return readLine()!.split(separator: " ").map { Int($0)! }
    }
    
    let S = Array(readLine()!)
    let T = Array(readLine()!)
    
    //print((0..<S.count).compactMap { S.index[$0] == T.index[$0] ? 0 : 1 }.reduce(0) { $0 + $1 })
    print((0..<S.count).map { S[$0] == T[$0] ? 0 : 1 }.reduce(0, +))
}

main()
