int main()
{
	int a[10],b[10],n,i,m=0;
	float GPA,c[10],sum=0.0;
	scanf("%d",&n);
	for(i=0;i<n;i++)
	{
		scanf("%d",&a[i]);
		m=m+a[i];
	}
	for(i=0;i<n;i++)
	{
		scanf("%d",&b[i]);
		if(b[i]>=90&&b[i]<=100)
			c[i]=4.0;
		if(b[i]>=85&&b[i]<=89)
			c[i]=3.7;
		if(b[i]>=82&&b[i]<=84)
			c[i]=3.3;
		if(b[i]>=78&&b[i]<=81)
			c[i]=3.0;
		if(b[i]>=75&&b[i]<=77)
			c[i]=2.7;
		if(b[i]>=72&&b[i]<=74)
			c[i]=2.3;
		if(b[i]>=68&&b[i]<=71)
			c[i]=2.0;
		if(b[i]>=64&&b[i]<=67)
			c[i]=1.5;
		if(b[i]>=60&&b[i]<=63)
			c[i]=1.0;
		if(b[i]<60)
			c[i]=0.0;
	}
	for(i=0;i<n;i++)
		sum=sum+a[i]*c[i];
    GPA=sum/m;
    printf("%.2f\n",GPA);
	return 0;
}