import java.util.*; class Main { static class Node implements Comparable { 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 queue = new PriorityQueue(); 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 * (c + exp) < exp * 1e-3 ) { exp = exp + rem * (c + exp); 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); } }