#include <iostream>
#include <cmath>
#include <iomanip>
using namespace std;

int main(){
    double px, py, qx, qy;
    cin >> px >> py >> qx >> qy;
    double res = (abs(py - qy) + abs(px -qx)) / 2;
    cout << setprecision(15) << res << endl;

    return 0;
}