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

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