#include "iostream"
#include "math.h"
using namespace std;

int x[2],y[2];
int ans;
int main(){
cin >>x[0]>>y[0]>>x[1]>>y[1];
ans+=abs(x[0]-x[1])+abs(y[0]-y[1]);
if(ans%2==0)cout <<ans/2;
else cout <<ans/2<<".5";
cout <<"\n";
return 0;
}