#include <stdio.h>
int main(){
  int p1,p2,q1,q2,l1,l2;
  scanf("%d %d\n%d %d\n",&p1,&p2,&q1,&q2);
  l1=q1-p1;l2=q2-p2;
  if(0>(q1-p1))l1=-l1;
  if(0>(q2-p2))l2=-l2;
  printf("%.1f\n",(l1+l2)/2.0);
  return 0;
}