#include "bits/stdc++.h"
using namespace std;
#define int long long
#define REP(i,n) for(int i = 0;i < (int)(n);i++)
#define RREP(i,n) for(int i = (int)n-1;i >= 0;i--)
#define FOR(i,s,n) for(int i = s;i < (int)n;i++)
#define RFOR(i,s,n) for(int i = (int)n-1;i >= s;i--)
#define ALL(a) a.begin(),a.end()
template<class T>inline void out(T t){cout<<t<<"\n";}
template<class T,class... Ts>inline void out(T t,Ts... ts){cout<<t<<" ";out(ts...);}
template<class T>inline bool CHMAX(T&a,T b){if(a<b){a = b;return true;}return false;}
template<class T>inline bool CHMIN(T&a,T b){if(a>b){a = b;return true;}return false;}
constexpr long long INF = 1e18;

int gcd(int x,int y)
{
	if(x==0) return y;
	return gcd(y%x,x);
}
int lcm(int x, int y)
{
	return (x * y / gcd(x, y));
}

signed main(){
	int A,B;
	cin >> A >> B;
	if(gcd(A,B) != 1){
		out(-1);
		return 0;
	}
	vector<int>dp(A*B+1);
	dp[0] = 1;
	REP(i,A*B+1){
		if(dp[i]&&i+A<A*B+1)dp[i+A]=1;
	}
	REP(i,A*B+1){
		if(dp[i]&&i+B<A*B+1)dp[i+B]=1;
	}
	out(A*B-(accumulate(ALL(dp),0)-1));
}