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

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

  scanf("%lf %lf %lf %lf",&x1,&y1,&x2,&y2);
  x=x2-x1;
  y=y2-y1;
  re=sqrt(x*x+y*y);
  printf("%f\n",re);

  return 0;
}