import std.algorithm, std.container, std.conv, std.math, std.range, std.typecons, std.stdio, std.string;

auto rdsp(){return readln.splitter;}
void pick(R,T)(ref R r,ref T t){t=r.front.to!T;r.popFront;}
void readV(T...)(ref T t){auto r=rdsp;foreach(ref v;t)pick(r,v);}

void main()
{
  long n; readV(n);
  long s; readV(s);

  auto calc(long n, long b) {
    auto s = 0L;
    while (n >= b) {
      s += n % b;
      n /= b;
    }
    return s+n;
  }

  auto t = n-s;
  if (t <= 0) {
    writeln(-1);
    return;
  }

  long[] d;
  foreach (di; 1..t.nsqrt+1)
    if (t%di == 0) d ~= [di, t/di];

  d.sort();
  foreach (di; d) {
    auto b = di+1;
    if (s == calc(n, b)) {
      writeln(b);
      return;
    }
  }

  writeln(-1);
}

pure T nsqrt(T)(T n)
{
  import std.algorithm, std.conv, std.range, core.bitop;
  if (n <= 1) return n;
  T m = T(1) << (n.bsr/2+1);
  return iota(1, m).map!"a * a".assumeSorted!"a <= b".lowerBound(n).length.to!T;
}
