#include <iostream>
#include<vector>
#include<algorithm>
#include <map>
#define ll long long
using namespace std;
int main()
{
    int n;
    cin >> n;
    vector<int> a;
    a.resize(n);
    map<ll, ll>mp;
    for (int i = 0; i < n; i++)
    {
        cin >> a[i];
        mp[a[i]]++;
    }

    ll pow[32];
    pow[0] = 1;
    for (int i = 1; i < 32; i++) pow[i] = pow[i - 1] * 2;

    ll ans = 0;
    for (int i = 0; i < n; i++)
    {
        mp[a[i]]--;
        for (int j = 0; j < 32; j++)
        {
            ll x = pow[j] - a[i];
            if (x > 0 && mp[x] > 0)
                ans += mp[x];
        }
    }
    cout << ans << endl;
}