#include <stdio.h>
#include <math.h>
  
int main(void){
  
  double x1,x2,y1,y2;
  double dis = 0;
  
  scanf("%lf %lf %lf %lf",&x1,&y1,&x2,&y2);
  dis = ((x1 - x2) * (x1 - x2)) + ((y1 - y2) * (y1 - y2));
  dis = sqrt(dis);
  printf("%f\n",dis);
  return 0;
}