double ass(int n, double b)
{
	int i;
    double a[16],temp,db;
    a[1] = exp(-b) / b;
    if( n == 1)  goto yi;
    db = 1.0 / b;
    for (i = 2;i<=n;i++)
	{
        a[i] = a[1] + db * (i - 1) * a[i - 1];
	}
yi:  temp = a[n];
	 return temp;
}