#include<stdio.h>
#include<math.h>
int main(){
  double x[2],y[2],kyori,datax,datay,tate,yoko;
  scanf("%lf %lf %lf %lf",&x[0],&x[1],&y[0],&y[1]);
  if(x[0]>y[0])yoko=x[0]-y[0];
  else yoko=y[0]-x[0];
  if(x[1]>y[0])tate=x[1]-y[1];
  else tate=y[1]-x[1];
  kyori=hypot(tate,yoko);
  printf("%f\n",kyori);
  return(0);
}
  