#include <stdio.h>
#include <math.h>
int main(void){
    // Your code here!
    double x1,y1,x2,y2;
    double d;
    
    scanf("%lf %lf %lf %lf", &x1, &y1, &x2, &y2);
    
    d=(x2-x1)*(x2-x1)+(y2-y1)*(y2-y1);
    
    printf("%.8lf", sqrt(d));
    
    return 0;
}

