int main(){
	int n;
	scanf("%d",&n);
	int a[11],b[11];
	int i;
	double c[11];
	double k;
	k=0.0;
	double p;
	p=0.0;
	for(i=0;i<n;i++){
		scanf("%d",&a[i]);
		p=p+a[i];
	}
	for(i=0;i<n;i++){
		scanf("%d",&b[i]);
		if(b[i]>=90&&b[i]<=100){
			c[i]=4.0;
		}
		else if(b[i]>=85&&b[i]<=89){
			c[i]=3.7;
		}
		else if(b[i]>=82&&b[i]<=84){
			c[i]=3.3;
		}
		else if(b[i]>=78&&b[i]<=81){
			c[i]=3.0;
		}
		else if(b[i]>=75&&b[i]<=77){
			c[i]=2.7;
		}
		else if(b[i]>=72&&b[i]<=74){
			c[i]=2.3;
		}
		else if(b[i]>=68&&b[i]<=71){
			c[i]=2.0;
		}
		else if(b[i]>=64&&b[i]<=67){
			c[i]=1.5;
		}
		else if(b[i]>=60&&b[i]<=63){
			c[i]=1.0;
		}
		else if(b[i]<60){
			c[i]=0.0;
		}
		k=k+c[i]*a[i];
	}
	double GPA;
	GPA=k/p;
	printf("%.2lf",GPA);
	return 0;
}