#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
using namespace std;

int main() {
    int px,py,qx,qy;
    cin >> px >> py >> qx >> qy;
    double dx,dy;
    dx = px - qx;
    dy = py - qy;
    dx *= dx < 0 ? -1 : 1;
    dy *= dy < 0 ? -1 : 1;
    printf("%f",dx/2 + dy/2);
    return 0;
}