#include <stdio.h>
#include <math.h>
int main(){    
double x1, y1, x2, y2, distance;    
scanf("%lf %lf %lf %lf", &x1, &y1, &x2, &y2);    
distance=sqrt((pow(x2-x1, 2)+pow(y2-y1, 2)));    
printf("%f\n", distance);    
return 0;}
