#include <bits/stdc++.h>
using namespace std;
constexpr int INF = numeric_limits<int>::max() / 2;
constexpr long long LINF = numeric_limits<long long>::max() / 3;
struct Double {
  double d;
  explicit Double(double x) : d(x) {}
};
ostream& operator<<(ostream& os, const Double x) {
  os << fixed << setprecision(20) << x.d;
  return os;
}
template <typename T>
ostream& operator<<(ostream& os, const vector<T>& vec) {
  os << "[";
  for (const auto& v : vec) {
    os << v << ",";
  }
  os << "]";
  return os;
}
template <typename T, typename U>
ostream& operator<<(ostream& os, const pair<T, U>& p) {
  os << "(" << p.first << "," << p.second << ")";
  return os;
}
template <typename T>
ostream& operator<<(ostream& os, const set<T>& st) {
  os << "{";
  for (T v : st) os << v << ",";
  os << "}";
  return os;
}
template <typename T, typename U>
inline void chmax(T& x, U y) {
  if (y > x) x = y;
}
template <typename T, typename U>
inline void chmin(T& x, U y) {
  if (y < x) x = y;
}
long long gcd(long long a, long long b) {
  if (b == 0)
    return a;
  else
    return gcd(b, a % b);
}
constexpr double eps = 1e-10;
constexpr long long mod = 1e9 + 7;
const int dx[] = {1, 0, -1, 0}, dy[] = {0, 1, 0, -1};
long long f(long long b, long long n) {
  if (n < b)
    return n;
  else
    return f(b, (n / b)) + (n % b);
}
long long sq(long long n) {
  for (long long i = 1; i < n; i++) {
    if (i * i > n) return i - 1;
  }
}
int main() {
  long long n, s;
  cin >> n >> s;
  long long sqn = sq(n);
  for (long long i = 2; i <= sqn; i++) {
    if (f(i, n) == s) {
      cout << i << endl;
      return 0;
    }
  }
  if (n < s) {
    cout << -1 << endl;
    return 0;
  }
  long long sqns = sq(n - s);
  for (long long x = sqns; x > 0; x--) {
    if ((n - s) % x) continue;
    long long b = (n - s) / x + 1;
    long long y = s - x;
    if (b < 2) continue;
    if (b > sqn and x < b and y >= 0 and y < b) {
      cout << b << endl;
      return 0;
    }
  }
  if (s == n and n > 1) {
    cout << n << endl;
    return 0;
  }
  cout << -1 << endl;
  return 0;
}
