int main()
{
	int n,i;
	float GPA,total=0.0,xuefen[20],score[20],zongfen=0.0;
	scanf("%d",&n);
	for(i=0;i<n;i++)
	{
		scanf("%f",&xuefen[i]);
        zongfen=zongfen+xuefen[i];
	}
	for(i=0;i<n;i++)
	{
		scanf("%f",&score[i]);
	}
	for(i=0;i<n;i++)
	{
		if(score[i]>=90&&score[i]<=100)
		{
			score[i]=4.0;
			continue;
		}
		if(score[i]>=85&&score[i]<=89)
		{
			score[i]=(float)3.7;
			continue;
		}
		if(score[i]>=82&&score[i]<=84)
		{
			score[i]=(float)3.3;
			continue;
		}
		if(score[i]>=78&&score[i]<=81)
		{
			score[i]=3.0;
			continue;
		}
		if(score[i]>=75&&score[i]<=77)
		{
			score[i]=(float)2.7;
			continue;
		}
		if(score[i]>=72&&score[i]<=74)
		{
			score[i]=(float)2.3;
			continue;
		}
		if(score[i]>=68&&score[i]<=71)
		{
			score[i]=2.0;
			continue;
		}
		if(score[i]>=64&&score[i]<=67)
		{
			score[i]=1.5;
			continue;
		}
		if(score[i]>=60&&score[i]<=63)
		{
			score[i]=1.0;
			continue;
		}
		if(score[i]<60)
		{
			score[i]=0.0;
			continue;
		}
	}
	for(i=0;i<n;i++)
	{
		total=score[i]*xuefen[i]+total;
	}
	GPA=total/zongfen;
	printf("%.2f\n",GPA);
	return 0;
}
