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