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

int main(void){
  double x1,x2,y1,y2;
  double d;
  scanf("%lf %lf %lf %lf",&x1,&y1,&x2,&y2);
  d=sqrt(pow((x2-x1),2)+pow((y2-y1),2));
  printf("%lf\n",d);
  return 0;
}
