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

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

	scanf("%lf %lf %lf %lf", &x1, &y1, &x2, &y2);
	
	kyo	= ((x1 - x2)*(x1 - x2)) + ((y1 - y2)*(y1 - y2));
	kyo = sqrt(kyo);//平方根の計算

	printf("%f", kyo);

	return 0;
}

