void main() {
	const A = readInt;
	const B = readInt;
	if (gcd(A, B) > 1) {
		writeln(-1);
		return;
	}
	auto dp = new bool[A * B];
	dp[0] = true;
	foreach (i; 0 .. A * B) {
		if (i + A < A * B) {
			dp[i + A] = dp[i + A] || dp[i];
		}
		if (i + B < A * B) {
			dp[i + B] = dp[i + B] || dp[i];
		}
	}
	writeln(dp.count(false));
}

import std,core.bitop;
string[]_R;
string readString(){while(_R.empty){_R=readln.chomp.split;}auto ret=_R.front;_R.popFront;return ret;}
int readInt(){return readString.to!int;}
long readLong(){return readString.to!long;}
real readReal(){return readString.to!real;}
bool chmin(T)(ref T A,T B){if(A>B){A=B;return true;}else{return false;}}
bool chmax(T)(ref T A,T B){if(A<B){A=B;return true;}else{return false;}}
int lowerBound(T)(T[]A,T x){int L=-1,R=cast(int)A.length;while(R-L>1){int mid=(L+R)/2;(A[mid]<x?L:R)=mid;}return R;}
int upperBound(T)(T[]A,T x){int L=-1,R=cast(int)A.length;while(R-L>1){int mid=(L+R)/2;(A[mid]<=x?L:R)=mid;}return R;}