#include <bits/stdc++.h>
#define REP(i,n) for(int i = 0; i < (int)(n); ++i)
using namespace std;

int main() {
    int Px, Py;
    int Qx, Qy;
    cin >> Px >> Py;
    cin >> Qx >> Qy;
    printf("%.1f\n",(abs(Px-Qx)+abs(Py-Qy))/2.0);
    return 0;
}