int main()
{
	int n[10],score[10];
	int k,i,m;
	double jd[10];
	double a,sum;
	m=0;
	sum=0.0;
	scanf("%d",&k);
	for(i=0;i<k;i++)
	{
		scanf("%d",&n[i]);
		m+=n[i];
	}
	i=0;
	while(i<k)
	{
		scanf("%d",&score[i]);
		if(score[i]>=90)
		{
			jd[i]=4.0;
		}else
		{
			if((score[i]>=85)&&(score[i]<90))
			{
				jd[i]=3.7;
			}else
			{
				if((score[i]>=82)&&(score[i]<85))
				{
					jd[i]=3.3;
				}else
				{
					if((score[i]>=78)&&(score[i]<82))
					{
						jd[i]=3.0;
					}else
					{
						if((score[i]>=75)&&(score[i]<78))
						{
							jd[i]=2.7;
						}else
						{
							if((score[i]>=72)&&(score[i]<75))
							{
								jd[i]=2.3;
							}else
							{
								if((score[i]>=68)&&(score[i]<72))
								{
									jd[i]=2.0;
								}else
								{
									if((score[i]>=64)&&(score[i]<68))
									{
										jd[i]=1.5;
									}else
									{
										if((score[i]>=60)&&(score[i]<64))
										{
											jd[i]=1.0;
										}else
										{
											jd[i]=0.0;
										}
									}
								}
							}
						}
					}
				}
			}
		}
		sum+=(jd[i])*(n[i]);
		i++;
	}
	a=sum/((double)(m));
	printf("%.2lf",a);
	return 0;
}