Problem p04016 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using V = vector<int>;
using Vll = vector<ll>;
using Vld = vector<ld>;
using Vbo = vector<bool>;
using VV = vector<V>;
using VVll = vector<Vll>;
using VVld = vector<Vld>;
using VVbo = vector<Vbo>;
using P = pair<int, int>;
using Pll = pair<ll, ll>;
using Pld = pair<ld, ld>;
#define rep2(i, m, n) for (ll i = int(m); i < int(n); ++i)
#define drep2(i, m, n) for (ll i = int(m) - 1; i >= int(n); --i)
#define rep(i, n) rep2(i, 0, n)
#define drep(i, n) drep2(i, n, 0)
#define all(a) a.begin(), a.end()
struct fast_ios {
  fast_ios() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(20);
  };
} fast_ios_;
template <typename T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &p) {
  is >> p.first >> p.second;
  return is;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
  os << "(" << p.first << ", " << p.second << ")";
  return os;
}
template <typename T> istream &operator>>(istream &is, vector<T> &v) {
  for (auto &e : v)
    is >> e;
  return is;
}
template <typename T> ostream &operator<<(ostream &os, const vector<T> &v) {
  for (auto &e : v)
    os << e << " ";
  return os;
}
template <typename T> inline int count_between(vector<T> &a, T l, T r) {
  return lower_bound(all(a), r) - lower_bound(all(a), l);
} // [l, r)
inline int Log2(ll x) {
  int k;
  for (k = 0; x > 0; ++k)
    x >>= 1;
  return k;
} // number of binary digits
const int INF = 1 << 30;
const ll INFll = 1ll << 62;
const ld EPS = 1e-10;
const int MOD = int(1e9) + 7;

ll rec(ll b, ll n) {
  if (n < b)
    return n;
  else
    return rec(b, n / b) + n % b;
}

int main() {
  ll n, s;
  cin >> n >> s;

  ll b;
  for (b = 2; b * b <= n; ++b) {
    if (s == rec(b, n)) {
      cout << b << "\n";
      return 0;
    }
  }
  for (ll k = b - 1; k > 0; --k) {
    b = (n + k - s) / k;
    if (s == rec(b, n)) {
      cout << b << "\n";
      return 0;
    }
  }

  if (s == n)
    cout << n + 1 << "\n";
  else
    cout << -1 << "\n";

  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using V = vector<int>;
using Vll = vector<ll>;
using Vld = vector<ld>;
using Vbo = vector<bool>;
using VV = vector<V>;
using VVll = vector<Vll>;
using VVld = vector<Vld>;
using VVbo = vector<Vbo>;
using P = pair<int, int>;
using Pll = pair<ll, ll>;
using Pld = pair<ld, ld>;
#define rep2(i, m, n) for (ll i = int(m); i < int(n); ++i)
#define drep2(i, m, n) for (ll i = int(m) - 1; i >= int(n); --i)
#define rep(i, n) rep2(i, 0, n)
#define drep(i, n) drep2(i, n, 0)
#define all(a) a.begin(), a.end()
struct fast_ios {
  fast_ios() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(20);
  };
} fast_ios_;
template <typename T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &p) {
  is >> p.first >> p.second;
  return is;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
  os << "(" << p.first << ", " << p.second << ")";
  return os;
}
template <typename T> istream &operator>>(istream &is, vector<T> &v) {
  for (auto &e : v)
    is >> e;
  return is;
}
template <typename T> ostream &operator<<(ostream &os, const vector<T> &v) {
  for (auto &e : v)
    os << e << " ";
  return os;
}
template <typename T> inline int count_between(vector<T> &a, T l, T r) {
  return lower_bound(all(a), r) - lower_bound(all(a), l);
} // [l, r)
inline int Log2(ll x) {
  int k;
  for (k = 0; x > 0; ++k)
    x >>= 1;
  return k;
} // number of binary digits
const int INF = 1 << 30;
const ll INFll = 1ll << 62;
const ld EPS = 1e-10;
const int MOD = int(1e9) + 7;

ll rec(ll b, ll n) {
  if (n < b)
    return n;
  else
    return rec(b, n / b) + n % b;
}

int main() {
  ll n, s;
  cin >> n >> s;

  ll b;
  for (b = 2; b * b <= n; ++b) {
    if (s == rec(b, n)) {
      cout << b << "\n";
      return 0;
    }
  }
  for (ll k = b - 1; k > 0; --k) {
    b = (n + k - s) / k;
    if (b < 2)
      continue;
    if (s == rec(b, n)) {
      cout << b << "\n";
      return 0;
    }
  }

  if (s == n)
    cout << n + 1 << "\n";
  else
    cout << -1 << "\n";

  return 0;
}