hdu 2401 Baskts of Gold Coins

/*  1--N-1          ,weight=w*(1+n-1)*(n-1)/2
       W,(weight-W)/d          */
#include <stdio.h>

int main()
{
	int n,w,d,W,ans;
	while(scanf("%d %d %d %d",&n,&w,&d,&W)!=EOF)
	{
		ans=(w*n*(n-1)/2-W)/d;
		if(ans>0)
			printf("%d
",ans);
		else
			printf("%d
",n);
	}
	return 0;
}