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

class Main{
    void solve(PrintWriter out , In in) {
        // A * B < C -> A < C / B
        int n = in.nextInt() , m = in.nextInt() ;
        long c = in.nextLong();
        long [] a = in.LongArray(n);
        long [] b = in.LongArray(m);
        Arrays.sort(a);
        Arrays.sort(b);
        long bottom = n * m ;
        long ok = 0 ;
        for(int i = 0 ; i < m ; i ++ ) {
            int p = upper_bound(a , c / b[i]);
            ok += (n - p) ;
        }
        out.print((double) ok / bottom);
    }
    int upper_bound(long [] A , long key) {
        int left = 0;
        int right = A.length;
        while(left < right) {
            int mid = (left + right) / 2;
            if(A[mid] <= key) left = mid + 1;
            else right = mid;
        }
        return right;
    }
    public static void main(String[] args) {
        PrintWriter pw = new PrintWriter(System.out);
        In in = new In();
        new Main().solve(pw,in);
        pw.flush();
    }
}

class Pair<T extends Comparable<T>, U extends Comparable<U>> implements Comparable<Pair<T, U>> {

    private final T first;
    private final U second;

    Pair(T first, U second) {
        this.first = first;
        this.second = second;
    }

    T first() {
        return first;
    }

    U second() {
        return second;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Pair<?, ?> pair = (Pair<?, ?>) o;
        return Objects.equals(first, pair.first) && Objects.equals(second, pair.second);
    }

    @Override
    public int hashCode() {
        return Objects.hash(first, second);
    }

    @Override
    public int compareTo(Pair<T, U> other) {
        T small_First  = this.first  , large_First  = other.first;
        U small_Second = this.second , large_Second = other.second;
        int First_Result  = small_First.compareTo(large_First);
        int Second_Result = small_Second.compareTo(large_Second);
        return First_Result == 0 ? Second_Result : First_Result ;
    }

    @Override
    public String toString() {
        return this.first+" "+this.second;
    }
}
class Triple<T extends Comparable<T>, U extends Comparable<U>, V extends Comparable<V>> implements Comparable<Triple<T, U, V>> {
    
    private final T first;
    private final U second;
    private final V third;

    Triple(T first, U second, V third) {
        this.first = first;
        this.second = second;
        this.third = third;
    }

    T first() {
        return first;
    }

    U second() {
        return second;
    }

    V third() {
        return third;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Triple<?, ?, ?> triple = (Triple<?, ?, ?>) o;
        return Objects.equals(first, triple.first) 
            && Objects.equals(second, triple.second)
            && Objects.equals(third, triple.third) ;
    }


    @Override
    public int hashCode() {
        return Objects.hash(first, second, third);
    }

    @Override
    public int compareTo(Triple<T, U, V> other) {
        T small_First  = this.first  , large_First  = other.first;
        U small_Second = this.second , large_Second = other.second;
        V small_Third  = this.third  , large_Third  = other.third;
        int First_Result  = small_First.compareTo(large_First);
        int Second_Result = small_Second.compareTo(large_Second);
        int Third_Result  = small_Third.compareTo(large_Third);
        return First_Result == 0 ? (Second_Result == 0 ? Third_Result : Second_Result ) : First_Result;
    }
}
class In{
    private final InputStream in = System.in;
    private final byte[] buffer = new byte[1024];
    private int ptr = 0;
    private int buflen = 0;
    private boolean hasNextByte() {
        if (ptr < buflen) {
            return true;
        }else{
            ptr = 0;
            try {
                buflen = in.read(buffer);
            } catch (IOException e) {
                e.printStackTrace();
            }
            if (buflen <= 0) {
                return false;
            }
        }
        return true;
    }
    private int readByte() { 
        if (hasNextByte()) return buffer[ptr++]; else return -1;
    }
    private static boolean isPrintableChar(int c) { 
        return 33 <= c && c <= 126;
    }
    public boolean hasNext() { 
        while(hasNextByte() && !isPrintableChar(buffer[ptr])) {
            ptr++; 
        }
        return hasNextByte();
    }
    String next() {
        if (!hasNext()) throw new NoSuchElementException();
        StringBuilder sb = new StringBuilder();
        int b = readByte();
        while(isPrintableChar(b)) {
            sb.appendCodePoint(b);
            b = readByte();
        }
        return sb.toString();
    }

    long nextLong() {
        if (!hasNext()) throw new NoSuchElementException();
        long n = 0;
        boolean minus = false;
        int b = readByte();
        if (b == '-') {
            minus = true;
            b = readByte();
        }
        if (b < '0' || '9' < b) {
            throw new NumberFormatException();
        }
        while(true){
            if ('0' <= b && b <= '9') {
                n *= 10;
                n += b - '0';
            }else if(b == -1 || !isPrintableChar(b)){
                return minus ? -n : n;
            }else{
                throw new NumberFormatException();
            }
            b = readByte();
        }
    }
    int nextInt() {
        long nl = nextLong();
        if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE) throw new NumberFormatException();
        return (int) nl;
    }
    double nextDouble() { 
        return Double.parseDouble(next());
    }
    char nextChar() {
        return next().charAt(0);
    }
    int [] IntArray(int n) {
        final int [] Array = new int [n];
        for(int i = 0 ; i < n ; i ++ ) {
            Array[i] = nextInt();
        }
        return Array;
    }
    int [][] IntArray(int n , int m) {
        final int [][] Array = new int [n][m];
        for(int i = 0 ; i < n ; i ++ ) {
            Array[i] = IntArray(m);
        }
        return Array;
    }   
    long [] LongArray(int n) {
        final long [] Array = new long [n];
        for(int i = 0 ; i < n ; i ++ ) {
            Array[i] = nextLong();
        }
        return Array;
    }
    long [][] LongArray(int n , int m) {
        final long [][] Array = new long [n][m];
        for(int i = 0 ; i < n ; i ++ ) {
            Array[i] = LongArray(m);
        }
        return Array;
    }
    String [] StringArray(int n) {
        final String [] Array = new String [n];
        for(int i = 0 ; i < n ; i ++ ) {
            Array[i] = next();
        }
        return Array;
    }
    char [] CharArray(int n) {
        final char [] Array = new char[n];
        for(int i = 0 ; i < n ; i ++ ) {
            Array[i] = next().charAt(0);
        }
        return Array;
    }
    char [][] CharArray(int n , int m) {
        final char [][] Array = new char [n][m];
        for(int i = 0 ; i < n ; i ++ ) {
            Array[i] = next().toCharArray();
        }
        return Array;
    }
    char [][] CharArray2(int n , int m) {
        final char [][] Array = new char [n][m];
        for(int i = 0 ; i < n ; i ++ ) {
            for(int j = 0 ; j < n ; j ++ ) {
                Array[i][j] = next().charAt(0);
            }
        }
        return Array;
    }
}