#include <stdio.h>
#include <math.h>
double dlugosc(int a, int b, int c, int d)
{
    double x, y;
    x = a - b;
    y = c - d;
    return hypot(x, y);
}
int main()
{
    double a, b, c, d;
    scanf("%lf%lf%lf%lf", &a, &b, &c, &d);

    printf("%lf",dlugosc(a, b, c, d) );
    return 0;
}