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