#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define ALL(v) v.begin(),v.end()
typedef long long ll;
 
#include <bits/stdc++.h>
using namespace std;

const ll INF=1e9+7;

ll c;
bool f(ll a,ll x){
  if(x==INF) return true;
  else if(a*x>c) return true;
  else return false;
}

int main(){
  ll n,m;
  cin>>n>>m>>c;
  
  vector<ll> A(n+1),B(m+2);
  rep(i,n) cin>>A[i+1];
  rep(i,m) cin>>B[i+1];
  B[m+1]=INF;
  sort(ALL(B));
  
  ll cnt=0;
  for(int i=1;i<=n;i++){
    int ng=0,ok=m+1;
    while(ok-ng>1){
      int mid=ng+(ok-ng)/2;
      if(f(A[i],B[mid])) ok=mid;
      else ng=mid;
    }
    cnt+=m-ng;
  }
  cout<<fixed<<setprecision(10)<<(double)cnt/(n*m)<<endl;
  
  return 0;
}