#include <stdio.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; 
} 

