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

int main(void)
{
  double x1,x2,y1,y2;
  double distance;

  scanf("%lf%lf%lf%lf",&x1,&y1,&x2,&y2);
  
  distance = ((x2 - x1) * (x2 - x1))+((y2 - y1) * (y2 - y1));

  printf("%f\n",sqrt(distance));
  
  return(0);
}