#include <bits/stdc++.h>
using namespace std;

#define llong long long 
#define len(x) ((int)x.size())
#define rep(i,n) for (int i = -1; ++ i < n; )
#define rep1(i,n) for (int i = 0; i ++ < n; )
#define rand __rand
mt19937 rng(chrono::system_clock::now().time_since_epoch().count());  // or mt19937_64
template<class T = int> T rand(T range = numeric_limits<T>::max()) { return (T)(rng() % range); }

#define CONCAT_(x, y) x##y/*{{{*/
#define CONCAT(x, y) CONCAT_(x, y)
#ifdef LOCAL_DEBUG   
int __db_level = 0;
bool __db_same_line = false;
#define clog cerr << string(!__db_same_line ? __db_level * 2 : 0, ' ')
struct debug_block {
    function<void()> fn;
    void print_name() { __db_same_line = true; fn(); clog << endl; __db_same_line = false;  }
    debug_block(function<void()> fn_): fn(fn_) { clog << "{ "; print_name(); ++__db_level; }
    ~debug_block() { --__db_level; clog << "} "; print_name(); }
};
#define DB(args...) debug_block CONCAT(dbbl, __LINE__)([=]{ clog << args; })
#define deb(...)  if (1) { (clog << "[" #__VA_ARGS__ "] = [" << __VA_ARGS__) << "]"; if (!__db_same_line) clog << endl; }
#else
#define clog if (0) cerr
#define DB(...)
#define deb(...)
#endif
template<class T>
ostream& operator,(ostream& out, const T& thing) { return out << ", " << thing; }
template<class U, class V>
ostream& operator<<(ostream& out, const pair<U, V>& p) { return (out << "(" << p.first, p.second) << ")"; }
template<class A, class B>
ostream& operator<<(ostream& out, const tuple<A, B>& t) { return (out << "(" << get<0>(t), get<1>(t)) << ")"; }
template<class A, class B, class C>
ostream& operator<<(ostream& out, const tuple<A, B, C>& t) { return (out << "(" << get<0>(t), get<1>(t), get<2>(t)) << ")"; }
template<class T> ostream& operator<<(ostream& out, const vector<T>& container) { 
    out << "{";
    if (len(container)) out << container[0];
    rep1(i, len(container) - 1) out, container[i];
    return out << "}";
}
template<class x> vector<typename x::value_type> $v(const x& a) { return vector<typename x::value_type>(a.begin(), a.end()); }
#define ptrtype(x) typename iterator_traits<x>::value_type
template<class u> vector<ptrtype(u)> $v(u a, u b) { return vector<ptrtype(u)>(a, b); }/*}}}*/
// ACTUAL SOLUTION BELOW ////////////////////////////////////////////////////////////

llong n, s;

llong f(llong b, llong tn = n) {
    if (tn < b) return tn;
    return f(b, tn / b) + (tn % b);
}

int main(void) {
    ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    cin >> n >> s;
    if (s > n)  {
        cout << -1;
        return 0;
    }
    if (s == n) { 
        cout << n + 1; 
        return 0; 
    } 
    
    for (int i = 2; i <= 1000000; ++i) {
        if (f(i, n) == s) {
            cout << i;
            return 0;
        }
    }
    
    llong ans = n + 1;
    
    for (llong sd = 1; sd * sd <= n - s; ++sd) {
        if ((n - s) % sd) continue;
        for (llong d: {sd, (n - s) / sd}) {
            llong x = (n - s) / d + 1;
            deb(d, x, f(x, n));
            if (x >= 2 and f(x, n) == s) ans = min(ans, x);
        }
    }
    if (ans == n + 1) cout << -1;
    else cout << ans;

    return 0;
}

// Remember:
// - Multitest? REFRESHING the data!!!
// - Constrains for each set of data may differs. Should NOT USE the same max constant (maxn)
//   for all of them.
// vim: foldmethod=marker


Measured memory_bytes: 3648