#include <stdio.h>
#include <string>
#include <cstring>
#include <stdlib.h>
#include <math.h>
#include <algorithm>
#include <vector>
#include <set>
#include <map>
#include <queue>
#include <stack>
#include <list>
#include <iterator>
#include <assert.h>
#pragma warning(disable:4996) 
 
typedef long long ll;
#define MIN(a, b) ((a)>(b)? (b): (a))
#define MAX(a, b) ((a)<(b)? (b): (a))
#define LINF 9223300000000000000
#define INF 2140000000
const long long MOD = 1000000007;
//const long long MOD = 998244353;

using namespace std;

ll gcd(ll a, ll b) {
    if(b == 0) return a;
    return gcd(b,a%b);
}

void solve()
{
    int a,b;
    scanf("%d%d", &a, &b);
    if(gcd(a,b)>1) {
        printf("-1\n"); return;
    }
    vector<int> v(a*b*2);
    int i,j;
    for(i=0; i<b; i++) {
        for(j=0; j<a; j++) {
            int tmp=a*i+b*j;
            if(tmp<a*b*2) v[tmp]=1;
        }
    }
    int cnt=0;
    for(i=1; i<a*b; i++) {
        if(v[i]==0) cnt++;
    }
    printf("%d\n", cnt);

    return;
}

int main(int argc, char* argv[])
{
#if 1
    solve();
#else
    int T; scanf("%d", &T);
    while(T--) {
        solve();
    }
#endif
    return 0;
}