Problem:
Score : 500 points

Problem Statement

For integers b (b \geq 2) and n (n \geq 1), let the function f(b,n) be defined as follows:

f(b,n) = n, when n < b

f(b,n) = f(b,\,{\rm floor}(n / b)) + (n \ {\rm mod} \ b), when n \geq b

Here, {\rm floor}(n / b) denotes the largest integer not exceeding n / b,
and n \ {\rm mod} \ b denotes the remainder of n divided by b.

Less formally, f(b,n) is equal to the sum of the digits of n written in base b.
For example, the following hold:

f(10,\,87654)=8+7+6+5+4=30

f(100,\,87654)=8+76+54=138

You are given integers n and s.
Determine if there exists an integer b (b \geq 2) such that f(b,n)=s.
If the answer is positive, also find the smallest such b.

Constraints

1 \leq n \leq 10^{11}

1 \leq s \leq 10^{11}

n,\,s are integers.

Input

The input is given from Standard Input in the following format:

n
s

Output

If there exists an integer b (b \geq 2) such that f(b,n)=s, print the smallest such b.
If such b does not exist, print -1 instead.

Sample Input 1

87654
30

Sample Output 1

10

Sample Input 2

87654
138

Sample Output 2

100

Sample Input 3

87654
45678

Sample Output 3

-1

Sample Input 4

31415926535
1

Sample Output 4

31415926535

Sample Input 5

1
31415926535

Sample Output 5

-1

Solution in C++14 (Clang 3.8.0) (judge verdict: Runtime Error)

// #include <bits/stdc++.h>
#include <iostream>  // cin, cout, string
#include <algorithm> // sort, min/max, upper/lower_bound, transform, count/copy_if
#include <vector>
#include <utility> // pair, swap
#include <functional>
#include <iomanip> // setprecision
#include <map>
#include <set> // multiset
#include <unordered_map>
#include <unordered_set>
#include <tuple>
#include <stack>
#include <queue> // priority_queue
#include <deque>
#include <bitset>
#include <array>
#include <list>
#include <forward_list>
#include <complex>
#include <ratio>
#include <cassert>
#include <numeric> // accumulate, itoa
#include <random>  // mt19937
#include <iterator>
#include <chrono>
#include <cstring>

using namespace std;

// #define int long long

using i64 = int64_t;
using ll = long long;
using ld = long double;

template <typename T>
using vec = vector<T>;
template <typename T>
using mat = vec<vec<T>>;
template <typename T>
using dic = unordered_map<string, T>;
template <typename T>
using pqg = priority_queue<T, vector<T>, greater<T>>; // dijkstra
// template<typename T, typename U> using P = pair<T, U>;
// template<typename... Args> using func = function<Args...>;
// template<typename T> using pq = priority_queue<T>;

using pii = pair<int, int>;
using ppi = pair<pii, int>;
using pip = pair<int, pii>;
using ppp = pair<pii, pii>;

using vi = vec<int>;
using mi = mat<int>; // Graph
using vp = vec<pii>;

constexpr int IINF = 0x3f3f3f3f;
constexpr long long LINF = 0x3f3f3f3f3f3f3f3fLL;
// constexpr int INF = sizeof(int) == sizeof(long long) ? LINF : IINF;
constexpr long long MOD = 1e9 + 7;

#define _overload(_1, _2, _3, name, ...) name
#define _rep1(i, a, b) for (int i = (a), _##i = (b); i < _##i; ++i)
#define _rep2(i, n) _rep1(i, 0, n)
#define _rep3(n) _rep2(i, n)
#define rep(...) _overload(__VA_ARGS__, _rep1, _rep2, _rep3)(__VA_ARGS__)
#define _rrep1(i, a, b) for (int i = (b)-1, _##i = (a); i >= _##i; --i)
#define _rrep2(i, n) _rrep1(i, 0, n)
#define _rrep3(n) _rrep2(i, n)
#define rrep(...) _overload(__VA_ARGS__, _rrep1, _rrep2, _rrep3)(__VA_ARGS__)

#define each(i, a) for (auto &&i : a)
#define each2(i, a) for (const auto &i : a)

#define pb push_back
// emplace_back(*) == push_back(constructor(*))
#define eb emplace_back
#define pob pop_back
#define pf push_front
#define pof pop_front

#define np next_permutation
#define pp prev_permutation

// make_pair(A, B) == pair<T, U>(A, B)
#define mp make_pair
// mt(...) == tie(...)
#define mt make_tuple

#define ft first
#define sd second

#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define sz(a) (int)(a).size()
#define rev(a) reverse(all(a))
#define rs(a, l) (a).resize(l)

#define upper(s) transform(all(s), (s).begin(), topper)
#define lower(s) transform(all(s), (s).begin(), tolower)

#define sum(a) accumulate(all(a), 0)
#define elif else if
#define unless(a) if (!(a))
#define qt(a) #a

#define endl '\n'
#define dec(n) fixed << setprecision(n)
#define zfl(n) setfill(‘0’) << setw(n)

template <typename T = int, T l = 0, T r = (T)MOD>
auto rnd = bind(uniform_int_distribution<T>(l, r), mt19937(chrono::steady_clock::now().time_since_epoch().count()));

template <typename T>
inline constexpr T gcd(T a, T b)
{
    if (a == 0 || b == 0)
        return 0;
    while (b)
    {
        a = a % b;
        swap(a, b);
    }
    return a;
}
template <typename T>
inline constexpr T extgcd2(T a, T b, T &x, T &y)
{
    x = 1;
    y = 0;
    while (b)
    {
        T t = a / b;
        a -= t * b;
        swap(a, b);
        x -= t * y;
        swap(x, y);
    }
    return a;
}
template <typename T>
inline constexpr T lcm(T a, T b) { return a / gcd(a, b) * b; }

template <typename T>
inline constexpr bool chmax(T &a, const T &b)
{
    if (a < b)
    {
        a = b;
        return true;
    }
    return false;
}
template <typename T>
inline constexpr bool chmin(T &a, const T &b)
{
    if (b < a)
    {
        a = b;
        return true;
    }
    return false;
}

template <typename A, typename B>
istream &operator>>(istream &is, pair<A, B> &p)
{
    is >> p.first;
    is >> p.second;
    return is;
}
template <typename A, typename B>
ostream &operator<<(ostream &os, const pair<A, B> &p)
{
    os << p.first << " " << p.second;
    return os;
}

template <typename T>
istream &operator>>(istream &is, vector<T> &v)
{
    for (auto &&x : v)
        is >> x;
    return is;
}
template <typename T>
ostream &operator<<(ostream &os, const vector<T> &v)
{
    for (int i = 0; i < (int)v.size(); ++i)
        cout << v[i] << (i + 1 != (int)v.size() ? " " : "");
    return os;
}

#define debug(x) cout << #x << " = " << (x) << " (L" << __LINE__ << ")" << endl
#define trace(...) __f(#__VA_ARGS__, __VA_ARGS__)
template <typename Arg>
void __f(const char *name, const Arg &arg)
{
    cerr << name << " : " << arg << endl;
}
template <typename Arg, typename... Args>
void __f(const char *names, const Arg &arg, const Args &... args)
{
    const char *comma = strchr(names + 1, ',');
    cerr.write(names, comma - names) << " : " << arg << " | ";
    __f(comma + 1, args...);
}

inline auto yes(const bool &c) { return c ? "yes" : "no"; }
inline auto Yes(const bool &c) { return c ? "Yes" : "No"; }
inline auto YES(const bool &c) { return c ? "YES" : "NO"; }
inline auto Possible(const bool &c) { return c ? "Possible" : "Impossible"; }
inline auto POSSIBLE(const bool &c) { return c ? "POSSIBLE" : "IMPOSSIBLE"; }

constexpr int lim = 100007; // 10^5+7

ll n, s;

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

signed main()
{
    // cin.tie(0);
    // cout.tie(0);
    // ios_base::sync_with_stdio(false);
    cin >> n >> s;
    if (s == n)
    {
        cout << n + 1 << endl;
        return 0;
    }
    if (n < s)
    {
        cout << -1 << endl;
        return 0;
    }

    for (ll i = 2; i * i < n; i++)
    {
        if (f(i, n) == s)
        {
            cout << i << endl;
            return 0;
        }
    }
    for (ll i = 1; i * i < n; i++)
    {

        ll b = (n - s) / i + 1;
        // cout << b << endl;
        if (f(b, n / b) == s)
        {
            cout << b << endl;
            return 0;
        }
    }

    cout << -1 << endl;
    return 0;
}