#include <stdio.h>
#include <math.h>
    
typedef struct point{
  double x;
  double y;
} point;
    
double distance(point p1,point p2){
  double temp;
  temp=(p1.x-p2.x)*(p1.x-p2.x)+(p1.y-p2.y)*(p1.y-p2.y);
  return sqrt(temp);
}
    
int main(void){
  point p1,p2;
  double d;
    
  scanf("%lf%lf%lf%lf",&p1.x,&p1.y,&p2.x,&p2.y);
  d=distance(p1,p2);
  printf("%f\n",d);
    
  return 0;
}