#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<algorithm>
#include<vector>
#include<string>
#include<sstream>
#include<iomanip>
#include<utility>
#include<cmath>
#include<set>
#include<list>
#include<queue>
#include<stack>
#include<deque>
#include<map>
#include<set>
#include<cstring>
#include<iterator>
#include<bitset>
#include<assert.h>
//#include<chrono>
//#include<time.h>
//#include<windows.h>
using namespace std;
 
//#define int ll
typedef long long ll;
typedef pair<int,int> P;
typedef pair<int,pair<int,int> > PP;
typedef vector<int> VI;
typedef vector<long long int> VL;
//static const int MOD = 1000000007;
static const int INF = 2147483647;
//static const long long INF = 9223372000000000000;
//static const long long INF = 9223372000000000000/2;
//static const int INF = 1000010000;
//int dx4[4] = {0,1,0,-1}, dy4[4] = {-1,0,1,0};
//int dx5[5] = {-1,0,0,0,1}, dy5[5] = {0,-1,0,1,0};
//int dx8[8] = {-1,0,1,1,1,0,-1,-1}, dy8[8] = {1,1,1,0,-1,-1,-1,0};
//int dx9[9] = {-1,0,1,1,1,0,-1,-1,0}, dy9[9] = {1,1,1,0,-1,-1,-1,0,0};
#define PB push_back
#define MP make_pair
#define MT make_tuple
#define FI first
#define SE second
#define NP next_permutation
#define PQ priority_queue
#define UB upper_bound
#define LB lower_bound
#define SZ(a) int((a).size())
#define LEN(a) int((a).length())
#define SORT(c) sort((c).begin(),(c).end())
#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define REP(i,x) for(int i=0;i<(int)(x);i++)
#define REP1(i,x) for(int i=1;i<=(int)(x);i++)
#define RREP(i,x) for(int i=((int)(x)-1);i>=0;i--)
#define RREP1(i,x) for(int i=((int)(x));i>0;i--)
#define ALL(x) (x).begin(),(x).end()
struct edge {int /*from,*/to,cost;};

signed main(){
    int x,y;
    cin >> x >> y;

    assert(-100<=x&&x<=100&&-100<=y&&y<=100);

    if(x%2==y%2){puts("-1");return 0;}

    return 0;
}