import std.algorithm, std.conv, std.range, std.stdio, std.string; void readV(T...)(ref T t){auto r=readln.splitter;foreach(ref v;t){v=r.front.to!(typeof(v));r.popFront;}} T[] readArray(T)(size_t n){auto a=new T[](n),r=readln.splitter;foreach(ref v;a){v=r.front.to!T;r.popFront;}return a;} T[] readArrayM(T)(size_t n){auto a=new T[](n);foreach(ref v;a)v=readln.chomp.to!T;return a;} void main() { int n, m; readV(n, m); if (n <= m) writeln(1); else if (n%2 == 1 || n/2 > m) writeln(-1); else writeln(2); }