int main()
{int a[10],b[10];
 float c[10],e;
 int n,i,f;
 scanf("%d",&n);
 for(i=0;i<n;i++)
    { 
     scanf("%d",&a[i]);               
    }
 for(i=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;  
      }
 c[0]=a[0]*c[0];
 for(i=1;i<n;i++)
  {
   e=c[i]=a[i]*c[i]+c[i-1];
   f=a[i]=a[i]+a[i-1]; 
  }
 printf("%.2f",e/f) ;
 scanf("%d",&n);
 return 0;
} 