#include <bits/stdc++.h>
#define REP(i, n) for(int i = 0;i < n;i++)
#define REPR(i, n) for(int i = n;i >= 0;i--)
#define FOR(i, m, n) for(int i = m;i < n;i++)
#define FORR(i, m, n) for(int i = m;i >= n;i--)
#define SORT(v, n) sort(v, v+n);
#define VSORT(v) sort(v.begin(), v.end());
#define VRSORT(v) sort(v.begin(), v.end(),greater<int>());
#define INF 999999999
#define MOD 1000000007
#define M_PI 3.14159265358979323846
#define ALL(X) (X).begin(),(X).end()

#ifdef __LOCAL
    #define DBG(X) cout << #X << " = " << (X) << endl;
    #define SAY(X) cout << (X) << endl;
#else
    #define DBG(X)
    #define SAY(X)
#endif

#ifdef __LOCAL
    #include <filesystem>
    namespace fs = std::filesystem;
#endif

using namespace std;

using ll = long long int;
using ull = unsigned long long int;
using ld = long double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;

int dx[]={1, 0, -1, 0, 0};
int dy[]={0, 1, 0, -1, 0};
const int INT_INF = (int)(2e9);
const ll  LL_INF = (ll)(2e18);
const int NIL = -1;
static mt19937 _g(time(nullptr));
 
std::string pad(int num){ char buffer[4]; std::snprintf(buffer, sizeof(buffer), "%03d", num); return buffer;}
inline ll randint(ll a, ll b) { ll w = (_g() << 31LL) ^ _g(); return a + w % (b - a + 1); }
inline void fast_io() { ios_base::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr); };
template<typename T> inline T sign(T x) { return T(x > 0) - T(x < 0); }
template<typename T, typename S> inline ostream& operator<<(ostream& os, const pair<T, S> p) { cout << "[" << p.first << ";" << p.second << "]"; return os; }
template<typename T, typename S> inline ostream& operator<<(ostream& os, const map<T, S> p) { for (auto el : p) cout << "[" << el.first << ";" << el.second << "]"; return os; }
template<typename T> inline ostream& operator<<(ostream& os, const vector<T>& v) { for (auto el : v) cout << el << " "; return os; }
template<typename T> inline vector<T> fetch_vec(int sz) { vector<T> ret(sz); for (auto& elem : ret) cin >> elem; return ret; }

ll N,S;

void input(){
  #ifdef __LOCAL
    fs::path p = __FILE__;
    fs::path input,output;
    input = output = p.parent_path();
    input += string("/input/") + string(p.stem()) + string(".txt");
    output += string("/output/") + string(p.stem()) + string(".txt");
    freopen(input.c_str(), "r", stdin);
    freopen(output.c_str(), "w", stdout);
  #endif
  cin >> N >> S;
}

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

int solve(){
  if(S==N) {cout << (N+1LL) << endl; return 0;}
  if(S>N) {cout << -1 << endl; return 0;}
  // if(S>(N+1)/2) {cout << -1 << endl; return 0;}
  // if(S==1) {cout << N << endl; return 0;}
  for (ll i = 2LL; i*i <= N; i++)
  {
    if(func(i,N)==S) {cout << i << endl; return 0;}
  }
  vector<ll> ans;
  for (ll q = 1LL; q*q <=N ; q++)
  {
    ll b = 1LL+(N-S)/q;
    // b*bはオーバーフローするのでダメ。
    if(b==1LL) continue;
    if(func(b,N)==S) {ans.push_back(b);}
  }
  DBG(ans)
  if(!ans.empty()){
    cout << *min_element(ans.begin(),ans.end()) << endl;
    return 0;
  }
  else cout << -1 << endl;
  return 0;  
}

ll test(ll n){
  map<ll,ll> cnt;
  for (ll i = 2; i <= min(n,(ll)1000); i++)
  {
    ll f=func(i,n);
    cnt[f]++;
    cout << f << "," <<  endl;
  }
  cout << cnt << endl;
  
  return 0;
}


int main()
{
  input();
  // test(99999515530);
  // cout << func(49999757765,99999515530) << endl;
  solve();
  return 0;
}


Measured memory_bytes: 384