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#include<bits/stdc++.h>
#define mp make_pair
#define pb push_back
#define int long long
using namespace std;
int n, a, ile[5009], wynik, cur, mod=1000000007, moz=1, sum;
vector<int> v;
int pot(int pod, int wyk){
	if(wyk==0)
		return 1;
	else if(wyk%2==0){
		long long wyn=(pot(pod, wyk/2))%mod;
		return (wyn*wyn)%mod;
	}
	else
		return (pod*pot(pod, wyk-1)%mod)%mod;
}
int32_t main(){
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
	//freopen("input.txt", "r", stdin);
	//freopen("output.txt", "w", stdout);
	cin>>n;
	for(int i=0; i<n; i++){
		cin>>a;
		if(!ile[a])
			v.pb(a);
		ile[a]++;
	}
	sort(v.begin(), v.end());
	for(int u:v){
		//cout<<u<<"\n";
		if(sum+1<u){
			cout<<"0\n";
			return 0;
		}
		cur=(moz*((pot(2, ile[u])-1+mod))%mod)%mod;
		wynik=(wynik+cur)%mod;
		moz=(moz*((pot(2, ile[u])-1+mod))%mod)%mod;
		if(sum==u)
			moz+=ile[u];
		sum+=u*ile[u];
	}
	cout<<wynik<<"\n";
	return 0;
}