import std.stdio, std.conv, std.string, std.array, std.range, std.algorithm, std.container;
import std.math, std.random, std.bigint, std.datetime, std.format;
void main(string[] args){ if(args.length > 1) if(args[1] == "-debug") DEBUG = 1; solve(); }
void log()(){ writeln(""); } void log(T, A ...)(T t, lazy A a){ if(DEBUG) write(t, " "), log(a); } bool DEBUG = 0; 
string read(){ static string[] ss; while(!ss.length) ss = readln.chomp.split; string res = ss[0]; ss.popFront; return res; }
// ----- ----- ----- ----- ----- ----- ----- ----- ----- ----- ----- ----- //

void solve(){
	int n = read.to!int;
	long[] as;
	foreach(i; 0 .. n) as ~= read.to!long;
	
	int[long] ac;
	foreach(a; as){
		if(a !in ac) ac[a] = 0;
		ac[a] += 1;
	}
	
	int ans;
	long[] aq = ac.keys;
	aq.sort!"a>b"();
	foreach(a; aq){
		long b = 1;
		while(b <= a) b *= 2;
		b -= a;
		if(b == a) ans += ac[a] / 2;
		else{ // b < a
			if(b in ac){
				if(ac[b] > ac[a]){
					ans += ac[a];
					ac[b] -= ac[a];
				}
				else{
					ans += ac[b];
					ac[b] = 0;
				}
			}
		}
	}
	
	ans.writeln;
	
}
