#include <iostream>
using namespace std;

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