//include
//------------------------------------------
#include <vector>
#include <list>
#include <map>
#include <climits>
#include <set>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>
#include <queue>
#include <random>
#include <fstream>

using namespace std;

//typedef
//------------------------------------------
typedef long long LL;
typedef vector<int> VI;
typedef vector<bool> VB;
typedef vector<char> VC;
typedef vector<double> VD;
typedef vector<string> VS;
typedef vector<LL> VLL;
typedef vector<VI> VVI;
typedef vector<VB> VVB;
typedef vector<VS> VVS;
typedef vector<VLL> VVLL;
typedef vector<VVI> VVVI;
typedef vector<VVLL> VVVLL;
typedef pair<int, int> PII;
typedef pair<LL, LL> PLL;
typedef pair<int, string> PIS;
typedef pair<string, int> PSI;
typedef pair<string, string> PSS;


//数値・文字列
//------------------------------------------
inline int toInt(string s) {
    int v;
    istringstream sin(s);
    sin >> v;
    return v;
}

inline LL toLongLong(string s) {
    LL v;
    istringstream sin(s);
    sin >> v;
    return v;
}

template<class T>
inline string toString(T x) {
    ostringstream sout;
    sout << x;
    return sout.str();
}

inline VC toVC(string s) {
    VC data(s.begin(), s.end());
    return data;
}

template<typename List>
void SPRIT(const std::string &s, const std::string &delim, List &result) {
    result.clear();
    string::size_type pos = 0;
    while (pos != string::npos) {
	string::size_type p = s.find(delim, pos);
	if (p == string::npos) {
	    result.push_back(s.substr(pos));
	    break;
	}
	else {
	    result.push_back(s.substr(pos, p - pos));
	}
	pos = p + delim.size();
    }
}

string TRIM(const string &str, const char *trimCharacterList = " \t\v\r\n") {
    string result;
    string::size_type left = str.find_first_not_of(trimCharacterList);
    if (left != string::npos) {
	string::size_type right = str.find_last_not_of(trimCharacterList);
	result = str.substr(left, right - left + 1);
    }
    return result;
}

template<typename T>
bool VECTOR_EXISTS(vector<T> vec, T data) {
    auto itr = std::find(vec.begin(), vec.end(), data);
    size_t index = distance(vec.begin(), itr);
    if (index != vec.size()) {
	return true;
    }
    else {
	return 0;
    }
}

#define UPPER(s) transform((s).begin(), (s).end(), (s).begin(), ::toupper)
#define LOWER(s) transform((s).begin(), (s).end(), (s).begin(), ::tolower)



//四捨五入 nLen=小数点第N位にする
//------------------------------------------

//切り上げ
double ceil_n(double dIn, int nLen) {
    double dOut;
    dOut = dIn * pow(10.0, nLen);
    dOut = (double)(int)(dOut + 0.9);
    return dOut * pow(10.0, -nLen);
}

//切り捨て
double floor_n(double dIn, int nLen) {
    double dOut;
    dOut = dIn * pow(10.0, nLen);
    dOut = (double)(int)(dOut);
    return dOut * pow(10.0, -nLen);
}

//四捨五入
double round_n(double dIn, int nLen) {
    double dOut;
    dOut = dIn * pow(10.0, nLen);
    dOut = (double)(int)(dOut + 0.5);
    return dOut * pow(10.0, -nLen);
}

//n桁目の数の取得
int take_a_n(int num, int n) {
    string str = toString(num);
    return str[str.length() - n] - '0';
}


//進数
//------------------------------------------

//"1111011" → 123
int strbase_2to10(const std::string &s) {
    int out = 0;
    for (int i = 0, size = s.size(); i < size; ++i) {
	out *= 2;
	out += ((int)s[i] == 49) ? 1 : 0;
    }
    return out;
}

//"123" → 1111011
int strbase_10to2(const std::string &s) {
    int binary = toInt(s);
    int out = 0;
    for (int i = 0; binary > 0; i++) {
	out = out + (binary % 2) * pow(static_cast<int>(10), i);
	binary = binary / 2;
    }
    return out;
}

//"ABC" 2748
int strbase_16to10(const std::string &s) {
    int out = stoi(s, 0, 16);
    return out;
}

//1111011 → 123
int intbase_2to10(int in) {
    string str = toString(in);
    return strbase_2to10(str);
}

//123 → 1111011
int intbase_10to2(int in) {
    string str = toString(in);
    return strbase_10to2(str);
}

int intbase_16to10(int in) {
    string str = toString(in);
    return strbase_16to10(str);
}

//123→ "7B"
string intbase_10to16(unsigned int val, bool lower = true) {
    if (!val)
	return std::string("0");
    std::string str;
    const char hc = lower ? 'a' : 'A';     // 小文字 or 大文字表記
    while (val != 0) {
	int d = val & 15;     // 16進数一桁を取得
	if (d < 10)
	    str.insert(str.begin(), d + '0');  //  10未満の場合
	else //  10以上の場合
	    str.insert(str.begin(), d - 10 + hc);
	val >>= 4;
    }
    return str;
}

//整数を2進数表記したときの1の個数を返す
LL bitcount64(LL bits) {
    bits = (bits & 0x5555555555555555) + (bits >> 1 & 0x5555555555555555);
    bits = (bits & 0x3333333333333333) + (bits >> 2 & 0x3333333333333333);
    bits = (bits & 0x0f0f0f0f0f0f0f0f) + (bits >> 4 & 0x0f0f0f0f0f0f0f0f);
    bits = (bits & 0x00ff00ff00ff00ff) + (bits >> 8 & 0x00ff00ff00ff00ff);
    bits = (bits & 0x0000ffff0000ffff) + (bits >> 16 & 0x0000ffff0000ffff);
    return (bits & 0x00000000ffffffff) + (bits >> 32 & 0x00000000ffffffff);
}



//comparison
//------------------------------------------
#define C_MAX(a, b) ((a)>(b)?(a):(b))
#define C_MIN(a, b) ((a)<(b)?(a):(b))
#define C_ABS(a, b) ((a)<(b)?(b)-(a):(a)-(b))



//container util
//------------------------------------------
#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define SZ(a) int((a).size())
#define EACH(i, c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define EXIST(s, e) ((s).find(e)!=(s).end())
#define COUNT(obj, v) count((obj).begin(), (obj).end(), v)
#define SEARCH(v, w) search((v).begin(), (v).end(), (w).begin(), (w).end())
#define B_SEARCH(obj, v) binary_search((obj).begin(), (obj).end(), v)
#define SORT(c) sort((c).begin(),(c).end())
#define RSORT(c) sort((c).rbegin(),(c).rend())
#define REVERSE(c) reverse((c).begin(), (c).end())
#define SUMI(obj) accumulate((obj).begin(), (obj).end(), 0)
#define SUMD(obj) accumulate((obj).begin(), (obj).end(), 0.)
#define SUMLL(obj) accumulate((obj).begin(), (obj).end(), 0LL)
#define SUMS(obj) accumulate((obj).begin(), (obj).end(), string())
#define UB(obj, n) upper_bound((obj).begin(), (obj).end(), n)
#define LB(obj, n) lower_bound((obj).begin(), (obj).end(), n)
#define PB push_back
#define MP make_pair




//input output
//------------------------------------------
#define GL(s) getline(cin, (s))
#define INIT std::ios::sync_with_stdio(false);std::cin.tie(0);
#define OUT(d) std::cout<<(d);
#define OUT_L(d) std::cout<<(d)<<endl;
#define FOUT(n, data) std::cout<<std::fixed<<std::setprecision(n)<<(data);
#define FOUT_L(n, data) std::cout<<std::fixed<<std::setprecision(n)<<(data)<<"\n";
#define EL() std::cout << "\n";
#define SHOW_VECTOR(v) {std::cerr << #v << "\t:";for(const auto& xxx : v){std::cerr << xxx << " ";}std::cerr << "\n";}
#define SHOW_MAP(v) {std::cerr << #v << endl; for(const auto& xxx: v){std::cerr << xxx.first << " " << xxx.second << "\n";}}

template<typename T>
std::istream &operator >> (std::istream &is, std::vector<T> &vec) {
    for (T &x : vec) is >> x;
    return is;
}

template<typename T>
std::ostream &operator<<(std::ostream &os, const std::vector<T> &vec) {
    for (const T &x : vec) os << x << " ";
    return os;
}


//repetition
//------------------------------------------
#define FOR(i, a, b) for(int i=(a);i<(b);++i)
#define RFOR(i, a, b) for(int i=(b)-1;i>=(a);--i)
#define REP(i, n)  FOR(i,0,n)
#define RREP(i, n) for(int i = n-1;i >= 0;i--)
#define FORLL(i, a, b) for(LL i=LL(a);i<LL(b);++i)
#define RFORLL(i, a, b) for(LL i=LL(b)-1;i>=LL(a);--i)
#define REPLL(i, n) for(LL i=0;i<LL(n);++i)
#define RREPLL(i, n) for(LL i=LL(n)-1;i>=0;--i)
#define FOREACH(x, v) for(auto &(x) : (v))
#define FORITER(x, v) for(auto (x) = (v).begin(); (x) != (v).end(); ++(x))


//constant
//--------------------------------------------
const double EPS = 1e-10;
const double PI = acos(-1.0);
const int MOD = 1000000007;
const int dx[] = { -1, 0, 1, 0 };
const int dy[] = { 0, 1, 0, -1 };


//math
//--------------------------------------------

//min <= aim <= max
template<typename T>
inline bool BETWEEN(const T aim, const T min, const T max) {
    if (min <= aim && aim <= max) {
	return true;
    }
    else {
	return false;
    }
}


template<class T>
inline T SQR(const T x) { return x * x; }

template<class T1, class T2>
inline T1 POW(const T1 x, const T2 y) {
    if (!y)return 1;
    else if ((y & 1) == 0) {
	return SQR(POW(x, y >> 1));
    }
    else return POW(x, y ^ 1) * x;
}


template<typename T>
constexpr T ABS(T x) {
    return x < 0 ? -x : x;
}

//partial_permutation nPr 順列
//first・・最初の数
//middle・・r(取り出す数)
//last・・n(全体数)
template<class BidirectionalIterator>
bool next_partial_permutation(BidirectionalIterator first, BidirectionalIterator middle, BidirectionalIterator last) {
    reverse(middle, last);
    return next_permutation(first, last);
}

//combination nCr 組み合わせ
//first1・・最初の数
//last1==first2・・r(取り出す数)
//last2・・n(全体数)
template<class BidirectionalIterator>
bool next_combination(BidirectionalIterator first1, BidirectionalIterator last1, BidirectionalIterator first2,
    BidirectionalIterator last2) {
    if ((first1 == last1) || (first2 == last2)) {
	return false;
    }
    BidirectionalIterator m1 = last1;
    BidirectionalIterator m2 = last2;
    --m2;
    while (--m1 != first1 && !(*m1 < *m2)) {
    }
    bool result = (m1 == first1) && !(*first1 < *m2);
    if (!result) {
	while (first2 != m2 && !(*m1 < *first2)) {
	    ++first2;
	}
	first1 = m1;
	std::iter_swap(first1, first2);
	++first1;
	++first2;
    }
    if ((first1 != last1) && (first2 != last2)) {
	m1 = last1;
	m2 = first2;
	while ((m1 != first1) && (m2 != last2)) {
	    std::iter_swap(--m1, m2);
	    ++m2;
	}
	std::reverse(first1, m1);
	std::reverse(first1, last1);
	std::reverse(m2, last2);
	std::reverse(first2, last2);
    }
    return !result;
}




//numeric_law
//--------------------------------------------

template<typename T>
constexpr bool ODD(T x) {
    return x % 2 != 0;
}

template<typename T>
constexpr bool EVEN(T x) {
    return x % 2 == 0;
}

//最大公約数
template<class T>
inline T GCD(const T x, const T y) {
    if (x < 0)return GCD(-x, y);
    if (y < 0)return GCD(x, -y);
    return (!y) ? x : GCD(y, x % y);
}

//最小公倍数
template<class T>
inline T LCM(const T x, const T y) {
    if (x < 0)return LCM(-x, y);
    if (y < 0)return LCM(x, -y);
    return x * (y / GCD(x, y));
}

//ax + by = 1
//x,yが変数に格納される
template<class T>
inline T EXTGCD(const T a, const T b, T &x, T &y) {
    if (a < 0) {
	T d = EXTGCD(-a, b, x, y);
	x = -x;
	return d;
    }
    if (b < 0) {
	T d = EXTGCD(a, -b, x, y);
	y = -y;
	return d;
    }
    if (!b) {
	x = 1;
	y = 0;
	return a;
    }
    else {
	T d = EXTGCD(b, a % b, x, y);
	T t = x;
	x = y;
	y = t - (a / b) * y;
	return d;
    }
}

//素数
template<class T>
inline bool ISPRIME(const T x) {
    if (x <= 1)return false;
    for (T i = 2; SQR(i) <= x; i++)if (x % i == 0)return false;
    return true;
}

//素数をtrueとして返す
template<class T>
VB ERATOSTHENES(const T n) {
    VB arr(n, true);
    for (int i = 2; SQR(i) < n; i++) {
	if (arr[i]) {
	    for (int j = 0; i * (j + 2) < n; j++) {
		arr[i * (j + 2)] = false;
	    }
	}
    }
    return arr;
}

// a <= x < b の素数を返す
template<typename T>
VB ERATOSTHENES(const T a, const T b) {
    VB small = ERATOSTHENES(b);
    VB prime(b - a, true);

    for (int i = 2; (T)(SQR(i)) < b; i++) {
	if (small[i]) {
	    for (T j = max(2, (a + i - 1) / i) * i; j < b; j += i) {
		prime[j - a] = false;
	    }
	}
    }

    return prime;
}

//約数
template<class T>
vector<T> DIVISOR(T n) {
    vector<T> v;
    for (int i = 1; i * i <= n; ++i) {
	if (n % i == 0) {
	    v.push_back(i);
	    if (i != n / i) {
		v.push_back(n / i);
	    }
	}
    }
    sort(v.begin(), v.end());
    return v;
}

//組み合わせ個数
template<typename T>
T NCR(T n, T r) {
    T ans = 1;
    REPLL(i, r) {
	ans = ans * (n - i) / (i + 1);
    }
    return ans;
}

//行列
int MATRIZ_CHAIN(VI &p, VVI &s) {
    const static int INF = 1 << 20;
    const int n = p.size() - 1;
    VVI X(n, VI(n, INF));
    s.resize(n, VI(n));
    for (int i = 0; i < n; ++i) X[i][i] = 0;
    for (int w = 1; w < n; ++w)
	for (int i = 0, j; j = i + w, j < n; ++i)
	    for (int k = i; k < j; ++k) {
		int f = p[i] * p[k + 1] * p[j + 1];
		if (X[i][k] + X[k + 1][j] + f < X[i][j]) {
		    X[i][j] = X[i][k] + X[k + 1][j] + f;
		    s[i][j] = k;
		}
	    }
    return X[0][n - 1];
}

//最長増加部分列
VI LIS(const VI &a) {
    const static int INF = 99999999;
    const int n = a.size();
    VI A(n, INF);
    VI id(n);
    for (int i = 0; i < n; ++i) {
	id[i] = distance(A.begin(), lower_bound(A.begin(), A.end(), a[i]));
	A[id[i]] = a[i];
    }
    int m = *max_element(id.begin(), id.end());
    VI b(m + 1);
    for (int i = n - 1; i >= 0; --i)
	if (id[i] == m) b[m--] = a[i];
    return b;
}

//最長共通部分列 string->toVC
template<typename T>
vector<T> LCS(const vector<T> &a, const vector<T> &b) {
    const int n = a.size(), m = b.size();
    vector<VI> X(n + 1, VI(m + 1));
    vector<VI> Y(n + 1, VI(m + 1));
    REP(i, n) {
	REP(j, m) {
	    if (a[i] == b[j]) {
		X[i + 1][j + 1] = X[i][j] + 1;
		Y[i + 1][j + 1] = 0;
	    }
	    else if (X[i + 1][j] < X[i][j + 1]) {
		X[i + 1][j + 1] = X[i][j + 1];
		Y[i + 1][j + 1] = +1;
	    }
	    else {
		X[i + 1][j + 1] = X[i + 1][j];
		Y[i + 1][j + 1] = -1;
	    }
	}
    }
    vector<T> c;
    for (int i = n, j = m; i > 0 && j > 0;) {
	if (Y[i][j] > 0) --i;
	else if (Y[i][j] < 0) --j;
	else {
	    c.PB(a[i - 1]);
	    --i;
	    --j;
	}
    }
    REVERSE(c);
    return c;
}

//コイン C総額 cs使用できるコインの種類
VI money_change(int C, VI &cs) {
    const int INF = 99999999;
    int n = cs.size();
    VI xs(C + 1, INF);
    VI ys(C + 1);
    xs[0] = 0;
    for (int i = 0; i < n; ++i) {
	for (int c = 0; c + cs[i] <= C; ++c) {
	    if (xs[c + cs[i]] > xs[c] + 1) {
		xs[c + cs[i]] = xs[c] + 1;
		ys[c + cs[i]] = c;
	    }
	}
    }
    VI zs;
    for (int c = C; c > 0; c = ys[c]) {
	zs.push_back(c - ys[c]);
    }
    return zs;
}


//Vector2D
//--------------------------------------------
template<class T>
struct Vec2D {
    T x, y;

    Vec2D() = default;

    constexpr Vec2D(T x, T y)
	: x{ x }, y{ y } {}

    static constexpr T Dot(const Vec2D &lhs, const Vec2D &rhs) {
	return lhs.x * rhs.x + lhs.y * rhs.y;
    }

    static constexpr T Cross(const Vec2D &lhs, const Vec2D &rhs) {
	return{ lhs.x * rhs.y - lhs.y * rhs.x };
    }

    constexpr T dot(const Vec2D &rhs) const {
	Dot(*this, rhs);
    }

    constexpr Vec2D cross(const Vec2D &rhs) const {
	Dot(*this, rhs);
    }

    T magnitude() const {
	return std::sqrt(sqrMagnitude());
    }

    constexpr T sqrMagnitude() const {
	return x * x + y * y;
    }

    Vec2D normalized() const {
	return *this / magnitude();
    }

    Vec2D normalize() {
	*this = normalized();
	return *this;
    }

    constexpr bool isZero() const {
	return x == 0 && y == 0;
    }

    constexpr Vec2D operator+() const {
	return *this;
    }

    constexpr Vec2D operator-() const {
	return{ -x, -y };
    }


    constexpr Vec2D operator+(Vec2D rhs) const {
	return{ x + rhs.x, y + rhs.y };
    }

    constexpr Vec2D operator-(Vec2D rhs) const {
	return{ x - rhs.x, y - rhs.y };
    }

    template<class U>
    constexpr Vec2D operator*(U rhs) const {
	return{ x * rhs, y * rhs };
    }

    template<class U>
    constexpr Vec2D operator/(U rhs) const {
	return{ x / rhs, y / rhs };
    }

    constexpr Vec2D &operator+=(const Vec2D &other) {
	x += other.x;
	y += other.y;
	return *this;
    }

    constexpr Vec2D &operator-=(const Vec2D &other) {
	x -= other.x;
	y -= other.y;
	return *this;
    }

    template<class U>
    constexpr Vec2D &operator*=(U s) {
	x *= s;
	y *= s;
	return *this;
    }

    template<class U>
    constexpr Vec2D &operator/=(U s) {
	x /= s;
	y /= s;
	return *this;
    }

};

template<class T, class U>
inline constexpr Vec2D<T> operator*(U s, Vec2D<T> v) {
    return{ s * v.x, s * v.y };
}

//Vector3D
//--------------------------------------------
template<class T>
struct Vec3D {
    T x, y, z;

    Vec3D() = default;

    constexpr Vec3D(T x, T y, T z)
	: x{ x }, y{ y }, z{ z } {}

    constexpr Vec3D(T x, T y)
	: Vec3D(x, y, 0) {}

    static constexpr T Dot(const Vec3D &lhs, const Vec3D &rhs) {
	return lhs.x * rhs.x + lhs.y * rhs.y + lhs.z * rhs.z;
    }

    static constexpr Vec3D Cross(const Vec3D &lhs, const Vec3D &rhs) {
	return{
	    lhs.y * rhs.z - lhs.z * rhs.y,
	    lhs.z * rhs.x - lhs.x * rhs.z,
	    lhs.x * rhs.y - lhs.y * rhs.x
	};
    }

    constexpr T dot(const Vec3D &rhs) const {
	Dot(*this, rhs);
    }

    constexpr Vec3D cross(const Vec3D &rhs) const {
	Dot(*this, rhs);
    }

    T magnitude() const {
	return std::sqrt(sqrMagnitude());
    }

    constexpr T sqrMagnitude() const {
	return x * x + y * y + z * z;
    }

    Vec3D normalized() const {
	return *this / magnitude();
    }

    Vec3D normalize() {
	*this = normalized();
	return *this;
    }

    constexpr bool isZero() const {
	return x == 0 && y == 0 && z == 0;
    }

    constexpr Vec3D operator+() const {
	return *this;
    }

    constexpr Vec3D operator-() const {
	return{ -x, -y, -z };
    }


    constexpr Vec3D operator+(Vec3D rhs) const {
	return{ x + rhs.x, y + rhs.y, z + rhs.z };
    }

    constexpr Vec3D operator-(Vec3D rhs) const {
	return{ x - rhs.x, y - rhs.y, z - rhs.z };
    }

    template<class U>
    constexpr Vec3D operator*(U rhs) const {
	return{ x * rhs, y * rhs, z * rhs };
    }

    template<class U>
    constexpr Vec3D operator/(U rhs) const {
	return{ x / rhs, y / rhs, z / rhs };
    }

    constexpr Vec3D &operator+=(const Vec3D &other) {
	x += other.x;
	y += other.y;
	z += other.z;
	return *this;
    }

    constexpr Vec3D &operator-=(const Vec3D &other) {
	x -= other.x;
	y -= other.y;
	z -= other.z;
	return *this;
    }

    template<class U>
    constexpr Vec3D &operator*=(U s) {
	x *= s;
	y *= s;
	z *= s;
	return *this;
    }

    template<class U>
    constexpr Vec3D &operator/=(U s) {
	x /= s;
	y /= s;
	z /= s;
	return *this;
    }

};

template<class T, class U>
inline constexpr Vec3D<T> operator*(U s, Vec3D<T> v) {
    return{ s * v.x, s * v.y, s * v.z };
}


//confirmation
//--------------------------------------------

//clear memory
#define CLR(arr, d) memset((arr), (d),sizeof(arr))

//debug
#define dump(x)  cerr << #x << " = " << (x) << endl;
#define debug(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl;


/*
*
*
*   ~~~~Below My Answer~~~~
*
*
**/


int main() {

    VB primes = ERATOSTHENES(1000000);
    primes[1] = false;

    while (true) {
	int n;
	cin >> n;
	
	if (n == 0) break;
	
	int count = 0;

	for (int i = n+1; i <= 2 * n; i++) {
	    if (primes[i]) count++;
	}

	OUT_L(count);

    }

    return 0;
}






































Measured memory_bytes: 3220