import java.io.IOException; import java.io.InputStream; import java.io.PrintWriter; import java.util.Arrays; import java.util.InputMismatchException; import java.util.NoSuchElementException; public class Main { static PrintWriter out; static InputReader ir; static void solve() { int n = ir.nextInt(); int m = ir.nextInt(); long c = ir.nextLong(); long[] a = ir.nextLongArray(n); long[] b = ir.nextLongArray(m); Arrays.sort(a); long res = 0; for (int i = 0; i < m; i++) { res += n - upper_bound(a, 0, n - 1, c / b[i]); } out.println(1.0 * res / n / m); } public static int upper_bound(long[] a, int fromIndex, int toIndex, long val) { if (val >= a[toIndex]) return toIndex + 1; if (val < a[fromIndex]) return fromIndex; int lb = fromIndex - 1, ub = toIndex; while (ub - lb > 1) { int mid = (ub - lb) / 2 + lb; if (a[mid] > val) { ub = mid; } else { lb = mid; } } return ub; } public static void main(String[] args) { ir = new InputReader(System.in); out = new PrintWriter(System.out); solve(); out.flush(); } static class InputReader { private InputStream in; private byte[] buffer = new byte[1024]; private int curbuf; private int lenbuf; public InputReader(InputStream in) { this.in = in; this.curbuf = this.lenbuf = 0; } public boolean hasNextByte() { if (curbuf >= lenbuf) { curbuf = 0; try { lenbuf = in.read(buffer); } catch (IOException e) { throw new InputMismatchException(); } if (lenbuf <= 0) return false; } return true; } private int readByte() { if (hasNextByte()) return buffer[curbuf++]; else return -1; } private boolean isSpaceChar(int c) { return !(c >= 33 && c <= 126); } private void skip() { while (hasNextByte() && isSpaceChar(buffer[curbuf])) curbuf++; } public boolean hasNext() { skip(); return hasNextByte(); } public String next() { if (!hasNext()) throw new NoSuchElementException(); StringBuilder sb = new StringBuilder(); int b = readByte(); while (!isSpaceChar(b)) { sb.appendCodePoint(b); b = readByte(); } return sb.toString(); } public int nextInt() { if (!hasNext()) throw new NoSuchElementException(); int c = readByte(); while (isSpaceChar(c)) c = readByte(); boolean minus = false; if (c == '-') { minus = true; c = readByte(); } int res = 0; do { if (c < '0' || c > '9') throw new InputMismatchException(); res = res * 10 + c - '0'; c = readByte(); } while (!isSpaceChar(c)); return (minus) ? -res : res; } public long nextLong() { if (!hasNext()) throw new NoSuchElementException(); int c = readByte(); while (isSpaceChar(c)) c = readByte(); boolean minus = false; if (c == '-') { minus = true; c = readByte(); } long res = 0; do { if (c < '0' || c > '9') throw new InputMismatchException(); res = res * 10 + c - '0'; c = readByte(); } while (!isSpaceChar(c)); return (minus) ? -res : res; } public double nextDouble() { return Double.parseDouble(next()); } public int[] nextIntArray(int n) { int[] a = new int[n]; for (int i = 0; i < n; i++) a[i] = nextInt(); return a; } public long[] nextLongArray(int n) { long[] a = new long[n]; for (int i = 0; i < n; i++) a[i] = nextLong(); return a; } public char[][] nextCharMap(int n, int m) { char[][] map = new char[n][m]; for (int i = 0; i < n; i++) map[i] = next().toCharArray(); return map; } } static void tr(Object... o) { System.err.println(Arrays.deepToString(o)); } }