#include <stdio.h>
#include <math.h>

int main(void)
{
    double x, y, x_, y_, dx, dy;

    scanf("%lf %lf %lf %lf", &x, &y, &x_, &y_);

    dx = x_ - x;
    dy = y_ - y;
    printf("%lf\n", sqrt(dx * dx + dy * dy));

    return 0;
}