#pragma GCC optimize("O3")
#include <bits/stdc++.h>
#define ll long long
#define rep(i,n) for(ll i=0;i<(n);i++)
#define pll pair<ll,ll>
#define pii pair<int,int>
#define pq priority_queue
#define pb push_back
#define eb emplace_back
#define fi first
#define se second
#define endl '\n'
#define ios ios_base::sync_with_stdio(0),cin.tie(0),cout.tie(0);
#define lb(c,x) distance(c.begin(),lower_bound(all(c),x))
#define ub(c,x) distance(c.begin(),upper_bound(all(c),x))

using namespace std;

inline int topbit(unsigned long long x){
	return x?63-__builtin_clzll(x):-1;
}

inline int popcount(unsigned long long x){
	return __builtin_popcountll(x);
}

inline int parity(unsigned long long x){//popcount%2
	return __builtin_parity(x);
}



template<class T> inline bool chmax(T& a,T b){if(a<b){a=b;return 1;}return 0;}
template<class T> inline bool chmin(T& a,T b){if(a>b){a=b;return 1;}return 0;}

const ll INF=1e18;


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(a.begin(),a.end());
    sort(b.begin(),b.end());
    double sum=0.0;
    rep(i,n){
        auto it=upper_bound(b.begin(),b.end(),c/a[i])-b.begin();
        sum+=(double)(m-it);
    }
    double p=n*m;
    sum/=p;
    cout << setprecision(10) << fixed;
    cout << sum << endl;
    return 0;
}