#include <iostream>
#include<map>
#include<vector>
#include<math.h>
using namespace std;
int main() {
    long long n;
    cin>>n;
    map<long long ,long long>mp;
    long long x;
    while(n--)
    {
        cin>>x;
        mp[x]++;
    }
    vector<long long>v;
    for(long long i=0;i<=32;i++)
    {
        v.push_back((long long )pow(2,i));
    }
    long long c=0;
 
 
            for(auto i:v)
            {
                for(auto k:mp)
                {
            if(mp.find(i-k.first)!=mp.end())
            {
                if(i-k.first==k.first &&mp[i-k.first]<=1){continue;}
                c+=mp[i-k.first];
                if(i-k.first!=k.first)
                {
                    auto it=i-k.first;
                    mp.erase(it);
 
                }
 
            }
        }
    }
    cout<<c;
    return 0;
}

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5
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