#include <iostream>
using namespace std;

int main(){
    int px = 0, py = 0;
    int qx = 0, qy = 0;
    cin >> px >> py;
    cin >> qx >> qy;
    printf("%f\n" ,double( abs(px - qx) + abs(py - qy) ) / 2.0);
    return 0;
}