Go: package main

import (
	"fmt"
	"math"
	"math/cmplx"
)

func main() {
	for {
		var n int
		fmt.Scan(&n)

		if n == 0 {
			return
		}

		arr := make([]complex128, n)

		for i := 0; i < n; i++ {
			var x, y float64

			fmt.Scanf("%f%f", &x, &y)

			arr[i] = complex(x, y)
		}

		max := 1
		for i := 0; i < n; i++ {
			for j := i + 1; j < n; j++ {
				sub := arr[j] - arr[i]

				if cmplx.Abs(sub) >= 2 {
					continue
				}
				sum := arr[j] + arr[i]

				v := 1
				{
					halfd := cmplx.Abs(sub / 2)

					center := sum/2 + sub*cmplx.Rect(math.Sqrt(1-halfd*halfd)/cmplx.Abs(sub), float64(v)*math.Pi/2)

					counter := 0
					for k := 0; k < n; k++ {
						if i == k || j == k || cmplx.Abs(arr[k]-center) <= 1 {
							counter++
						}
					}
					if max < counter {
						max = counter
					}
				}

				v = -1
				{
					halfd := cmplx.Abs(sub / 2)

					center := sum/2 + sub*cmplx.Rect(math.Sqrt(1-halfd*halfd)/cmplx.Abs(sub), float64(v)*math.Pi/2)

					counter := 0
					for k := 0; k < n; k++ {
						if i == k || j == k || cmplx.Abs(arr[k]-center) <= 1 {
							counter++
						}
					}
					if max < counter {
						max = counter
					}
				}

			}
		}

		fmt.Println(max)
	}
}

C++: #include<deque>
#include<queue>
#include<vector>
#include<algorithm>
#include<iostream>
#include<set>
#include<cmath>
#include<tuple>
#include<string>
#include<chrono>
#include<functional>
#include<iterator>
#include<random>
#include<unordered_set>
#include<array>
#include<map>
#include<iomanip>
#include<assert.h>
#include<bitset>
#include<stack>
#include<memory>
using namespace std;
using namespace std::chrono;
typedef long long int llint;
typedef double lldo;
#define mp make_pair
#define mt make_tuple
#define pub push_back
#define puf push_front
#define pob pop_back
#define pof pop_front
#define fir first
#define sec second
#define res resize
#define ins insert
#define era erase

/*
cout<<fixed<<setprecision(20);
cin.tie(0);
ios::sync_with_stdio(false);
*/
const llint mod=1000000007;
const llint big=2.19e15+1;
const long double pai=3.141592653589793238462643383279502884197;
const long double eps=1e-7;
template <class T,class U>bool mineq(T& a,U b){if(a>b){a=b;return true;}return false;}
template <class T,class U>bool maxeq(T& a,U b){if(a<b){a=b;return true;}return false;}
llint gcd(llint a,llint b){if(a%b==0){return b;}else return gcd(b,a%b);}
llint lcm(llint a,llint b){if(a==0){return b;}return a/gcd(a,b)*b;}
template<class T> void SO(T& ve){sort(ve.begin(),ve.end());}
template<class T> void REV(T& ve){reverse(ve.begin(),ve.end());}
template<class T>llint LBI(vector<T>&ar,T in){return lower_bound(ar.begin(),ar.end(),in)-ar.begin();}
template<class T>llint UBI(vector<T>&ar,T in){return upper_bound(ar.begin(),ar.end(),in)-ar.begin();}
double dis(double ax,double ay,double bx,double by){return (ax-bx)*(ax-bx)+(ay-by)*(ay-by);}
int count_num(vector<double>&zX,vector<double>&zY,double x,double y){
	int i,n,ans=0;n=zX.size();
	for(i=0;i<n;i++){
		if(dis(zX[i],zY[i],x,y)<=1+eps){ans++;}
	}
	return ans;
}
bool solve(void){
	int i,j,n,ans=0;cin>>n;
	if(n==0){return false;}
	vector<double>zX(n);
	vector<double>zY(n);
	for(i=0;i<n;i++){cin>>zX[i]>>zY[i];}
	for(i=0;i<n;i++){maxeq(ans,count_num(zX,zY,zX[i],zY[i]));}
	for(i=0;i<n;i++){
		for(j=i+1;j<n;j++){
			double kyo=sqrt(dis(zX[i],zY[i],zX[j],zY[j]));
			if(kyo>=2){continue;}
			double cx=(zX[i]+zX[j])/2;
			double cy=(zY[i]+zY[j])/2;
			double wari=sqrt(4-kyo*kyo)/kyo;
			maxeq(ans,count_num(zX,zY,cx+wari*(cy-zY[i]),cy-wari*(cx-zX[i])));
			maxeq(ans,count_num(zX,zY,cx-wari*(cy-zY[i]),cy+wari*(cx-zX[i])));
		}
	}
	cout<<ans<<endl;
	return true;
}
int main(void){
	while(solve()){}
	return 0;
}