#include<stdio.h>
#include<math.h>

int main(){
  double p,x,y,x1,y1,x2,y2;

  scanf("%lf%lf%lf%lf",&x1,&y1,&x2,&y2);

  if(x1>x2)x=x1-x2;
  else x=x2-x1;

  if(y1>y2)y=y1-y2;
  else y=y2-y1;

  p=sqrt(x*x+y*y);
  printf("%f\n",p);
  return 0;
}