import Foundation

private func readWords<T>(_ f: (Substring) -> T) -> [T] {
    return readLine()!.split(separator: " ").map(f)
}

prefix operator *
private prefix func *<T>(a: inout IndexingIterator<[T]>) -> T {
    return a.next()!
}

func mainB() {
    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)
}

mainB()
