#include <bits/stdc++.h>
using namespace std;
using lint = long long int;
using pint = pair<int, int>;
using plint = pair<lint, lint>;
struct fast_ios { fast_ios(){ cin.tie(0); ios::sync_with_stdio(false); cout << fixed << setprecision(20); }; } fast_ios_;
#define FOR(i, begin, end) for(int i=(begin),i##_end_=(end);i<i##_end_;i++)
#define IFOR(i, begin, end) for(int i=(end)-1,i##_begin_=(begin);i>=i##_begin_;i--)
#define REP(i, n) FOR(i,0,n)
#define IREP(i, n) IFOR(i,0,n)
template<typename T> istream &operator>>(istream &is, vector<T> &vec){ for (auto &v : vec) is >> v; return is; }
template<typename T> ostream &operator<<(ostream &os, const vector<T> &vec){ os << "["; for (auto v : vec) os << v << ","; os << "]"; return os; }
template<typename T> ostream &operator<<(ostream &os, const deque<T> &vec){ os << "deq["; for (auto v : vec) os << v << ","; os << "]"; return os; }
template<typename T> ostream &operator<<(ostream &os, const set<T> &vec){ os << "{"; for (auto v : vec) os << v << ","; os << "}"; return os; }
template<typename T> ostream &operator<<(ostream &os, const unordered_set<T> &vec){ os << "{"; for (auto v : vec) os << v << ","; os << "}"; return os; }
template<typename T> ostream &operator<<(ostream &os, const multiset<T> &vec){ os << "{"; for (auto v : vec) os << v << ","; os << "}"; return os; }
template<typename T> ostream &operator<<(ostream &os, const unordered_multiset<T> &vec){ os << "{"; for (auto v : vec) os << v << ","; os << "}"; return os; }
template<typename T1, typename T2> ostream &operator<<(ostream &os, const pair<T1, T2> &pa){ os << "(" << pa.first << "," << pa.second << ")"; return os; }
template<typename TK, typename TV> ostream &operator<<(ostream &os, const map<TK, TV> &mp){ os << "{"; for (auto v : mp) os << v.first << "=>" << v.second << ","; os << "}"; return os; }
template<typename TK, typename TV> ostream &operator<<(ostream &os, const unordered_map<TK, TV> &mp){ os << "{"; for (auto v : mp) os << v.first << "=>" << v.second << ","; os << "}"; return os; }
template<typename T> void ndarray(vector<T> &vec, int len) { vec.resize(len); }
template<typename T, typename... Args> void ndarray(vector<T> &vec, int len, Args... args) { vec.resize(len); for (auto &v : vec) ndarray(v, args...); }
template<typename T> bool mmax(T &m, const T q) { if (m < q) {m = q; return true;} else return false; }
template<typename T> bool mmin(T &m, const T q) { if (m > q) {m = q; return true;} else return false; }
template<typename T1, typename T2> pair<T1, T2> operator+(const pair<T1, T2> &l, const pair<T1, T2> &r) { return make_pair(l.first + r.first, l.second + r.second); }
template<typename T1, typename T2> pair<T1, T2> operator-(const pair<T1, T2> &l, const pair<T1, T2> &r) { return make_pair(l.first - r.first, l.second - r.second); }
#define dbg(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ") " << __FILE__ << endl;
#define FI first
#define SE second
#define ALL(x) (x).begin(), (x).end()
#define SZ(x) ((lint)(x).size())
#define POW2(n) (1LL << (n))

lint f(lint b, lint N)
{
    if (N == 0) return 0;
    else return f(b, N / b) + N % b;
}

void NO()
{
    puts("-1");
    exit(0);
}

vector<int> gen_primes(int N) // primes in [2, N]
{
    vector<int> ans;
    vector<int> alive(N + 1, 1);
    lint now = 2;
    for (; now * now <= N; now++)
    {
        if (alive[now]) ans.push_back(now);
        for (int t = now; t <= N; t += now) alive[t] = 0;
    }
    for (; now <= N; now++) if (alive[now]) ans.push_back(now);
    return ans;
}

map<lint, int> prime_factorization(lint N, const vector<int> &primes)
{
    map<lint, int> ans;
    for (auto v : primes)
    {
        while (!(N % v))
        {
            N /= v;
            ans[v]++;
        }
        if (N == 1) return ans;
    }
    ans[N]++;
    return ans;
    // exit(1);
}

vector<lint> divisors(lint n, const vector<int> &primes)
{
    map<lint, int> factor = prime_factorization(n, primes);
    vector<lint> now{1}, nxt;
    for (auto pa : factor)
    {
        nxt.clear();
        for (auto v : now)
        {
            for (int i = 0; i < pa.second + 1; i++)
            {
                nxt.push_back(v);
                v *= pa.first;
            }
        }
        swap(now, nxt);
    }
    return now;
}

int main()
{
    lint N, S;
    cin >> N >> S;
    if (S > N)
    {
        NO();
    }
    if (N == S)
    {
        cout << N + 1 << endl;
        return 0;
    }
    FOR(b, 2, 400000)
    {
        lint s = f(b, N);
        if (s == S)
        {
            cout << b << endl;
            return 0;
        }
    }

    auto primes = gen_primes(400000);

    auto div = divisors(N - S, primes);
    sort(ALL(div));
    for (lint d : div)
    {
        lint b = d + 1;
        lint p = (N - S) / d;
        lint q = S - p;
        if (q >= b or p >= b or q < 0) continue;
        cout << b << endl;
        return 0;
    }

    NO();
}


Measured memory_bytes: 2176