#include <stdio.h>
#include <math.h>
int main(void)
{
  double x1,y1,x2,y2;
  double ans;
  
  scanf("%lf",&x1);
  scanf("%lf",&y1);
  scanf("%lf",&x2);
  scanf("%lf",&y2);

  ans=(x1-x2)*(x1-x2)+(y1-y2)*(y1-y2);
  ans=sqrt(ans);
  printf("%f\n",ans);

  return 0;
}