// No.410 出会い
// https://yukicoder.me/problems/no/410
//

#include <iostream>
#include <cmath>
#include <iomanip>
using namespace std;
double solve(int px, int py, int qx, int qy);


int main() {
    int px, py;
    int qx, qy;
    cin >> px >> py;
    cin >> qx >> qy;
    double ans = solve(px, py, qx, qy);
    cout << setprecision(10) << ans << endl;
}

double solve(int px, int py, int qx, int qy) {
    int xdiff = abs(px - qx);
    int ydiff = abs(py - qy);
    return (xdiff + ydiff) / 2.0;
}