#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
using namespace std;
typedef long long int ll;
typedef pair<int,int> P;
#define yn {puts("YES");}else{puts("NO");}
#define MAX_N 200005

int main() {
    ll n, m, c;
    cin >> n >> m >> c;
    ll a[n] = {};
    rep(i,n) cin >> a[i];
    ll b[m];
    rep(i,m) cin >> b[i];
    sort(a,a+n);
    sort(b,b+m);
    ll cnt = 0;
    rep(i,m){
        cnt += n - (lower_bound(a,a+n,c/b[i]+1) - a);
    }
    cout << (double)cnt / (n*m) << endl;
    return 0;
}