import java.io.*;
import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner();
        int n = sc.nextInt();
        int m = sc.nextInt();
        long C = sc.nextLong();
        long[] typeA = new long[n];
        for (int i = 0; i < n; i++) {
            typeA[i] = sc.nextInt();
        }
        long[] typeB = new long[m];
        for (int i = 0; i < m; i++) {
            typeB[i] = sc.nextInt();
        }
        Arrays.sort(typeA);
        Arrays.sort(typeB);
        int right = m;
        double ans = 0;
        for (int i = 0; i < n; i++) {
            while (right >= 1 && typeA[i] * typeB[right - 1] > C) {
                right--;
            }
            ans += m - right;
        }
        System.out.println(ans / n / m);
    }
}

class Scanner {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    StringTokenizer st = new StringTokenizer("");
    
    public Scanner() throws Exception {
        
    }
    
    public int nextInt() throws Exception {
        return Integer.parseInt(next());
    }
    
    public long nextLong() throws Exception {
        return Long.parseLong(next());
    }
    
    public double nextDouble() throws Exception {
        return Double.parseDouble(next());
    }
    
    public String next() throws Exception {
        while (!st.hasMoreTokens()) {
            st = new StringTokenizer(br.readLine());
        }
        return st.nextToken();
    }
}