#include<iostream>
#include<vector>
#include<algorithm>
#include<cstdio>
#include<cmath>
#include<string>
using namespace std;
int main(){
    int PX,PY,QX,QY,X,Y;
    cin >> PX >> PY >> QX >> QY;
    X = QX-PX;
    Y = QY-QY;
    if(X<0) X*=-1;
    if(Y<0) Y*=-1;
   // cout << X <<" "<< Y <<endl;
    cout << (double)((X+Y)/2.0);
    return 0;
}