#include <bits/stdc++.h>
using namespace std;

//--------------------------------------------
#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define RFOR(i,a,b) for(int i=(b)-1;i>=(a);i--)
#define REP(i,n) FOR(i,0,n)
#define RREP(i,n) RFOR(i,0,n)
#define LL long long
#define INF INT_MAX/3

//--------------------------------------------
const double EPS = 1e-14;
const double PI  = acos(-1.0);

//--------------------------------------------

int main(){
  double px,py,qx,qy;
  scanf("%lf %lf %lf %lf",&px,&py,&qx,&qy);
  printf("%.1lf\n",abs(px - qx) / 2 + abs(py - qy) / 2);
}