#include <bits/stdc++.h>
using namespace std;
#include <boost/multiprecision/cpp_int.hpp>
typedef boost::multiprecision::cpp_int mp;


int main(){
    int N,M;
    long long C;
    cin>>N>>M>>C;
    vector<long long> A(N),B(M);
    for(int i=0;i<N;++i){
        cin>>A[i];
    }
    for(int i=0;i<M;++i){
        cin>>B[i];
    }
    sort(A.begin(),A.end());
    sort(B.begin(),B.end());
    long long ans=0;
    int a=0,b=M-1;
    while(a<N){
        if(b>=0 && A[a]*B[b]>C){
            --b;
        }else{
            ans+=M-b-1;
            ++a;
        }
    }
    cout<<(double)ans/(double)M/(double)N<<endl;
}