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

  return 0;
}
  

