 
double gc(int m); 
int main(){
    int n,s,i; 
    s=0; 
    scanf("%d",&n); 
    double gpa=0; 
    int a[n],b[n]; 
    for(i=0;i<=n-1;i++)
    {
                       scanf("%d",&a[i]);
                       s+=a[i]; 
    }
    for(i=0;i<=n-1;i++)
    {
                       scanf("%d",&b[i]);
                       gpa+=gc(b[i])*a[i]; 
    }
    gpa=gpa/s; 
    printf("%.2lf",gpa);
 
    return 0;
} 
double gc(int m)
{
       int v=1;
       int x=60; 
       while(x<101)
       {
             if(x<=m)v++;else break; 
             if((x<=68)||(x==78)) x+=4;
               else
               {if((x<78)||(x==82))x+=3;
                    else 
                    {if(x==85)x+=5; else 
                       {if(x==90)x=100;
                        else x=101;}
                    } 
               }

       } 
             
       switch(v) 
       {
              case 1: return 0;
              case 2:return 1.0;break; 
              case 3:return 1.5;break;
              case 4:return 2.0;break;
              case 5:return 2.3;break;
              case 6:return 2.7;break;
              case 7:return 3.0;break;
              case 8:return 3.3;break;
              case 9:return 3.7;break;
              case 10:return 4.0;break;
       }
} 
               
                               
