#include <bits/stdc++.h>

using namespace std;

typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef pair<int, int> pii;
typedef pair<long long, long long> PLL;
typedef pair<int, PII> TIII;

typedef long long ll;
typedef long double ld;
typedef unsigned long long ull;


#define FOR(i, s, n) for (int i = s; i < (int)n; ++i)
#define REP(i, n) FOR(i, 0, n)
#define rep(i, a, b) for (int i = a; i < (b); ++i)
#define trav(a, x) for (auto &a : x)
#define all(x) x.begin(), x.end()

#define MOD 1000000007

template<class T1, class T2> inline bool chmax(T1 &a, T2 b) {if (a < b) {a = b; return true;} return false;}
template<class T1, class T2> inline bool chmin(T1 &a, T2 b) {if (a > b) {a = b; return true;} return false;}
const double EPS = 1e-12, PI = acos(-1);
const double pi = 3.141592653589793238462643383279;
  
typedef string::const_iterator State;
ll GCD(ll a, ll b){
  return (b==0)?a:GCD(b, a%b);
}
ll LCM(ll a, ll b){
  return a/GCD(a, b) * b;
}
template< int mod >
struct ModInt {
  int x;

  ModInt() : x(0) {}

  ModInt(int64_t y) : x(y >= 0 ? y % mod : (mod - (-y) % mod) % mod) {}

  ModInt &operator+=(const ModInt &p) {
    if((x += p.x) >= mod) x -= mod;
    return *this;
  }

  ModInt &operator-=(const ModInt &p) {
    if((x += mod - p.x) >= mod) x -= mod;
    return *this;
  }

  ModInt &operator*=(const ModInt &p) {
    x = (int) (1LL * x * p.x % mod);
    return *this;
  }

  ModInt &operator/=(const ModInt &p) {
    *this *= p.inverse();
    return *this;
  }

  ModInt operator-() const { return ModInt(-x); }

  ModInt operator+(const ModInt &p) const { return ModInt(*this) += p; }

  ModInt operator-(const ModInt &p) const { return ModInt(*this) -= p; }

  ModInt operator*(const ModInt &p) const { return ModInt(*this) *= p; }

  ModInt operator/(const ModInt &p) const { return ModInt(*this) /= p; }

  bool operator==(const ModInt &p) const { return x == p.x; }

  bool operator!=(const ModInt &p) const { return x != p.x; }

  ModInt inverse() const {
    int a = x, b = mod, u = 1, v = 0, t;
    while(b > 0) {
      t = a / b;
      swap(a -= t * b, b);
      swap(u -= t * v, v);
    }
    return ModInt(u);
  }

  ModInt pow(int64_t n) const {
    ModInt ret(1), mul(x);
    while(n > 0) {
      if(n & 1) ret *= mul;
      mul *= mul;
      n >>= 1;
    }
    return ret;
  }

  friend ostream &operator<<(ostream &os, const ModInt &p) {
    return os << p.x;
  }

  friend istream &operator>>(istream &is, ModInt &a) {
    int64_t t;
    is >> t;
    a = ModInt< mod >(t);
    return (is);
  }

  static int get_mod() { return mod; }
};

using modint = ModInt< 1000000007 >;
template< typename T >
struct Combination {
  vector< T > _fact, _rfact, _inv;

  Combination(int sz) : _fact(sz + 1), _rfact(sz + 1), _inv(sz + 1) {
    _fact[0] = _rfact[sz] = _inv[0] = 1;
    for(int i = 1; i <= sz; i++) _fact[i] = _fact[i - 1] * i;
    _rfact[sz] /= _fact[sz];
    for(int i = sz - 1; i >= 0; i--) _rfact[i] = _rfact[i + 1] * (i + 1);
    for(int i = 1; i <= sz; i++) _inv[i] = _rfact[i] * _fact[i - 1];
  }

  inline T fact(int k) const { return _fact[k]; }

  inline T rfact(int k) const { return _rfact[k]; }

  inline T inv(int k) const { return _inv[k]; }

  T P(int n, int r) const {
    if(r < 0 || n < r) return 0;
    return fact(n) * rfact(n - r);
  }

  T C(int p, int q) const {
    if(q < 0 || p < q) return 0;
    return fact(p) * rfact(q) * rfact(p - q);
  }

  T H(int n, int r) const {
    if(n < 0 || r < 0) return (0);
    return r == 0 ? 1 : C(n + r - 1, r);
  }
};

ll modpow(ll x, ll n, ll mod) {
  ll res = 1;
  while(n) {
    if(n&1) res = (res * x) % mod;
    x = (x * x) % mod;
    n >>= 1;
  }
  return res;
}
typedef vector<ll> vec;
typedef vector<vec> mat;

mat mul(mat &A, mat &B){
  mat C(A.size(), vec(B[0].size()));
  for(int i=0; i<A.size(); i++){
    for(int k=0; k<B.size(); k++){
      for(int j=0; j<B[0].size(); j++){
        C[i][j] = (C[i][j] + A[i][k] % MOD * B[k][j] % MOD) % MOD;
        
      }
    }
  }
  return C;
}

mat matpow(mat A, ll n, ll mod = MOD) {
  mat B(A.size(), vec(A.size()));
  for(int i=0; i<A.size(); i++) B[i][i] = 1;

  while(n > 0){
    if(n&1) B = mul(B, A);
    A = mul(A, A);
    n >>= 1;
  }
  return B;
}
inline long long mod(long long a, long long m) {
    return (a % m + m) % m;
}
int main()
{
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(12);
  
  ll X; cin >> X;

  vector<ll> v;
  ll t = X;
  while(t){
    v.push_back(t%2);
    t/=2;
  }
  reverse(all(v));
  for(ll i=1; i<=v.size(); i++) {
    //bitの本数がiでX^i=Nであるような数が存在するか
    ll N = X^i;
    ll tar = 0;
    ll now = 0;
    for(int j=0; j<v.size(); j++) {
      if(tar == i) break;
      if(now + (1LL<<(v.size()-1-j)) > N) continue;
      now += (1LL<<(v.size()-1-j));
      tar++;
    }
    if(tar == i && now == N) {
      cout << N << endl;
      return 0;
    }
  }
  cout << -1 << endl;
  return 0;
}