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

int main(int argc, char** argv)
{
	double x1, y1, x2, y2, result;

	scanf("%lf %lf %lf %lf", &x1, &y1, &x2, &y2);

	double x = fabs(x1 - x2);
	double y = fabs(y1 - y2);

	result = sqrt((x * x) + (y * y));

	printf("%f\n", result);
	return 0;
}