import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.Arrays;

public class Main {
	public static void main(String[] args) throws Exception {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		String[] sa = br.readLine().split(" ");
		int n = Integer.parseInt(sa[0]);
		int m = Integer.parseInt(sa[1]);
		long c = Long.parseLong(sa[2]);
		sa = br.readLine().split(" ");
		long[] a = new long[n];
		for (int i = 0; i < n; i++) {
			a[i] = Integer.parseInt(sa[i]);
		}
		sa = br.readLine().split(" ");
		double[] b = new double[m];
		for (int i = 0; i < m; i++) {
			b[i] = Integer.parseInt(sa[i]);
		}
		br.close();

		long total = (long) n * m;
		Arrays.sort(a);
		Arrays.sort(b);
		long sum = 0;
		for (int i = 0; i < n; i++) {
			long val = c / a[i] + 1;
			int idx = Arrays.binarySearch(b, val - 0.5);
			if (idx < 0) idx = ~idx;
			sum += m - idx;
		}
		System.out.println((double) sum / total);
	}
}