#include<stdio.h>
#include<math.h>
int main(){
  int x1,x2,y1,y2;
  double len;

  scanf("%d %d",&x1,&y1);
  scanf("%d %d",&x2,&y2);
  x1 = 0;
  y1 = 0;
  x2 -= x1;
  y2 -= y1;

  len = sqrt((double)x2*x2 + y2*y2);
  printf("%f\n",len);
  return 0;
}

