#include <stdio.h>
#include <stdlib.h>

int main(){
  int ax, ay, bx, by;
  double time = 0;
  scanf("%d%d%d%d",&ax, &ay, &bx, &by);

  time += (double)abs(ax - bx)/2;
  time += (double)abs(ay - by)/2;

  printf("%f",time);
  return 0;
}