//  Created by conan1024hao in 2019.
//  Copyright © 2019 conan1024hao. All rights reserved.
//  専用ライブラリです、自由にコピーして構いません。
//  感谢看我的代码！Wechat:conan1024hao QQ:810396815
#pragma GCC optimize ("O3")
#include <iostream>
#include <iomanip>
#include <istream>
#include <ostream>
#include <sstream>
#include <iterator>
#include <vector>
#include <algorithm>
#include <queue>
#include <deque>
#include <list>
#include <stack>
#include <map>
#include <unordered_map>
#include <set>
#include <utility>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <string>
#include <ctime>
#include <cctype>
#include <cstdlib>
#define IINF 10e8
#define INF 10e16
#define MOD 1000000007
#define mod 1000000007
#define rep(i, a, n) for (ll i = a; i < (ll)(n); i++)
#define Endl endl
#define fi first
#define se second
#define pb push_back
#define mp make_pair
#define eb emplace_back
#define mmax(x,y)(x>y?x:y)
#define mmin(x,y)(x<y?x:y)
#define chmax(x,y) x=mmax(x,y)
#define chmin(x,y) x=mmin(x,y)
#define all(x) (x).begin(),(x).end()
#define siz(x) (ll)(x).size()
#define PI acos(-1.0)
using namespace std;
//using Int=int_fast64_t;
typedef long long int ll;
typedef pair<ll,ll>Pll;
typedef pair<int,int>Pin;
long long GCD(long long a, long long b) { return b ? GCD(b, a%b) : a; }
long long LCM(long long a, long long b)  {return a/GCD(a,b)*b;}
int dx[8]={-1,0,1,0,1,1,-1,-1};
int dy[8]={0,-1,0,1,1,-1,1,-1};
char dir[4]={'u','l','d','r'};
ll cmp1(pair<ll,string>a,pair<ll,string> b){
        if(a.fi!=b.fi)
        return a.fi<b.fi;
        else
        return a.se<b.se;
}
//--------------------------------------------------------------------------

//---------------------------------------------------------------------------
int main(){//問題をちゃんと見ろ!!!!!!!!!!!!!!!!!      llか??????????        memset()!!!!!!!!!!!!       ペナを減らせ!!!!!!!!!!!!!
    cin.tie(0);
    ios::sync_with_stdio(false);
    //-------------------------------
    int n;cin>>n;
    vector<int>a(n);
    for(int i=0;i<n;i++)cin>>a[i];
    int sum=0;double ave=0;
    for(int i=0;i<n;i++)sum+=a[i];
    ave=(double)sum/n;

    double minm=INF;
    int ans=0;
    for(int i=0;i<n;i++){
        if(minm>abs(a[i]-ave)){
            minm=abs(a[i]-ave);
            ans=i;
        }
    }
    cout<<ans<<endl;
    return 0;
    for(int i=0;i<n;i++){
        if(abs(a[i]-ave)==minm){
            cout<<i<<endl;
            return 0;
        }
    }
    //-------------------------------
    return 0;
}
//---------------------------------------------------------------------------








#include <bits/stdc++.h>
using namespace std;
using ll=long long;

int main() {
    set<ll> num;
    ll t = 1;
    while(t<ll(1e9)+5){
        t*=2;
        num.insert(t);
    }
    ll N;
    cin >> N;
    map<ll,ll> mp;
    vector<ll> A(N);
    for(int i=0;i<N;i++){
        cin >> A[i];
        mp[A[i]]++;
    }
    sort(A.rbegin(),A.rend());
    ll ans =0;
    for(int i=0;i<N;i++){
        if(mp[A[i]] ==0){
            continue;
        }
        ll n = A[i];
        ll t = *num.upper_bound(n);
        t-=n;
        mp[n]--;
        if(mp.find(t) == mp.end()|| mp[t] ==0){
            continue;
        }
        ans++;
        mp[t]--;
    }
   
    cout << ans << endl;
    return 0;
}