#include<iostream>
#include<string>
#include<vector>
#include<algorithm>
#include<bitset>
#include<set>
#include<map>
#include<stack>
#include<queue>
#include<deque>
#include<list>
#include<iomanip>
#include<cmath>
#include<cstring>
#include<functional>
#include<cstdio>
#include<cstdlib>
#include<numeric>

using namespace std;

#define repr(i, a, b) for (int i = (int)(a); i < (int)(b); i++)
#define rep(i, n) repr(i, 0, n)
#define INF 2e9
//#define MOD 1000000007
#define MOD 998244353
#define LINF (long long)4e18
#define jck 3.141592
#define PI acos(-1.0)

const double EPS = 1e-10;

using ll = long long;
using Pi = pair<int,int>;
using Pl = pair<ll,ll>;


int main(){
    ll n,m,c; cin >> n >> m >> c;
    vector<ll> a(n),b(m);
    rep(i,n) cin >> a[i];
    rep(i,m) cin >> b[i];
    sort(b.begin(),b.end());
    ll sum = 0;
    rep(i,n){
        int d = lower_bound(b.begin(),b.end(),c/a[i]+1)-b.begin();
        sum += m-d;
    }
    cout << fixed << setprecision(10);
    cout << (double)sum/(n*m) << endl;
}