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

int main(){
    double px, py, qx, qy;
    cin >> px >> py >> qx >> qy;
    if(px == qx){
        px = 0;
        qx = 0;
    }
    else if(py == qy){
        qy = 0;
        qy = 0;
    }
    cout << (abs(py - qy) + abs(px -qx)) / 2 << endl;

    return 0;
}