int main ()
{
	int n,a[10],b[10],i,s;
	float c[10],t;
	scanf("%d",&n);
	for(i=0,s=0;i<n;i++) 
	{
		scanf("%d",&a[i]);
		s+=a[i];
	}
	for(i=0,t=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;
        t+=c[i]*a[i];
	}
	printf("%.2f\n",t/s);
	return 0;
}

