#include<stdio.h>
int main()
{
	int xa, ya, xb, yb;
	scanf("%d %d", &xa, &ya);
	scanf("%d %d", &xb, &yb);
	xa *= -1;
	printf("%.20lf\n", ya - xa * (yb - ya) / (double)(xb - xa));
	return 0;
}