#include<iostream>
#include<vector>
#include<string>
#include<cstdio>
using namespace std;
int main(){
    int x1,x2,y1,y2,a,b;
    cin >> x1 >> y1 >> x2 >> y2;
    if(x1-x2>0) a=x1-x2;
    else a=(x1-x2)*-1;
    if(y1-y2>0) b=y1-y2;
    else b=(y1-y2)*-1;
    cout << (double)(a+b)/2;
    return 0;
}