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

typedef struct point{
  int x;
  int y;
} point;

double distance(point p1,point p2){
  int 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("%d%d%d%d",&p1.x,&p1.y,&p2.x,&p2.y);
  d=distance(p1,p2);
  printf("%.5f\n",d);

  return 0;
}