/*
 * ID:  ITP1_10_A
 * Problem: Distance
 */

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

int main(void)
{
    double x1, y1, x2, y2;

    scanf("%lf %lf %lf %lf", &x1, &y1, &x2, &y2);
    printf("%f\n", hypot(x1 - x2, y1 - y2));

    return 0;
}

