import std.stdio, std.conv, std.array, std.string, std.algorithm, std.container, std.range, core.stdc.stdlib, std.math, std.typecons;
T[][] combinations(T)(T[] s, in int m) {   if (!m) return [[]];   if (s.empty) return [];   return s[1 .. $].combinations(m - 1).map!(x => s[0] ~ x).array ~ s[1 .. $].combinations(m); }

void main() {
  auto AB = readln.split.to!(ulong[]);
  auto A = AB[0];
  auto B = AB[1];
  
  long solve() {
    if (A == 1 || B == 1) return 0;
    if (A % 2 == 0 && B % 2 == 0) return -1;

    bool[ulong] stock;
    auto smaller = A < B ? A : B;
    auto larger = A < B ? B : A;
    auto step = larger - smaller;
    foreach(n; 1..101) {
      foreach(i; iota(smaller*n, larger*n+1, step)) {
        stock[i] = true;
      }

      int continuas;
      foreach(i; smaller*(n-1)..larger*n) {
        if (!(i in stock)) {
          continuas = 0;
          continue;
        }
        if (++continuas == smaller) {
          int answer;
          foreach(j; 1..i) if (!(j in stock)) answer++;
          return answer;
        }
      }
    }

    return -1;
  }

  solve().writeln;
}