int main(int argc, char* argv[])
{int n,a[10],b[10],i,sum1=0;
 float c[10],d[10],sum2=0.0,GPA;
 scanf("%d",&n); 
 for(i=0;i<n;i++)
 {
	 scanf("%d",&a[i]);
     sum1+=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                        c[i]=0.0;
     d[i]=a[i]*c[i];
	 sum2+=d[i];
 }
 GPA=sum2/sum1;
 printf("%.2f",GPA);
	return 0;
}
