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

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[] valueA = new long[n];
        for (int i = 0; i < n; i++) {
            valueA[i] = sc.nextInt();
        }
        long[] valueB = new long[m];
        for (int i = 0; i < m; i++) {
            valueB[i] = sc.nextInt();
        }
        Arrays.sort(valueA);
        Arrays.sort(valueB);
        int right = 0;
        long ans = 0;
        for (int i = n - 1; i >= 0; i--) {
            while (right < m && valueA[i] * valueB[right] <= c) {
                right++;
            }
            ans += m - right;
        }
        System.out.println(new java.math.BigDecimal(1.0 * ans / m / n));
    }
}
class Utilities {
    static String arrayToLineString(Object[] arr) {
        return Arrays.stream(arr).map(x -> x.toString()).collect(Collectors.joining("\n"));
    }
    
    static String arrayToLineString(int[] arr) {
        return String.join("\n", Arrays.stream(arr).mapToObj(String::valueOf).toArray(String[]::new));
    }
}
class Scanner {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    StringTokenizer st = new StringTokenizer("");
    StringBuilder sb = new StringBuilder();
    
    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 int[] nextIntArray() throws Exception {
        return Stream.of(br.readLine().split(" ")).mapToInt(Integer::parseInt).toArray();
    }
    
    public String next() throws Exception {
        while (!st.hasMoreTokens()) {
            st = new StringTokenizer(br.readLine());
        }
        return st.nextToken();
    }
    
}