import java.util.*;

class Main {
  static class Node implements Comparable<Node> {
    double p;
    int i;
    public Node(double p, int i) {
      this.p = p;
      this.i = i;
    }
    public int compareTo(Node e) {
      if( this.p < e.p ) 
        return 1;
      if( this.p > e.p )
        return -1;
      return 0;
    }
  }
      
  public static void main(String[] args) {
    Scanner in = new Scanner(System.in);
    int n = in.nextInt();
    double[] p = new double[n];
    double[] q = new double[n];
    for( int i = 0; i < n; i++ ) {
      double x = in.nextInt();
      p[i] = x / 1000;
    }
    for( int i = 0; i < n; i++ ) {
      double x = in.nextInt();
      q[i] = x / 1000;
    }

    PriorityQueue<Node> queue = new PriorityQueue<Node>();
    for( int i = 0; i < n; i++ ) {
      if( p[i] > 0 ) {
        double x = p[i] * q[i];
        queue.add(new Node(x,i));
      }
    }
    double exp = 0;
    double rem = 1.0;
    int c = 1;
    while( !queue.isEmpty() ) {
      Node node = queue.poll();
      exp += c * node.p;
      rem -= node.p;
      c++;
      //System.err.println(node.i);
      if(  rem * n * 1000 < exp * 1e-5 ) {
        break;
      }
      /*
      if( c % 100000 == 0 ) {
        System.err.printf("%.4f %e %d\n",exp,rem,c);
      }
      */
      node.p *= 1 - q[node.i];
      queue.add(node);
    }
    System.out.printf("%5f\n",exp);
    //System.err.println(c);
    //System.err.printf("%e\n",rem);
  }
}