#include <iostream>
#include <cstdio>
#include <algorithm>
#include <vector>
#include <functional>
#include <queue>
#include <string>
#include <cstring>
#include <numeric>
#include <cstdlib>
#include <cmath>
#include <map>
#include <unordered_map>
#include <unordered_set>
using namespace std;
 
typedef long long ll;
 
using vi = vector<int>;
using vii = vector<vi>;
using vl = vector<ll>;
using vll = vector<vl>;

#define INF 10e17
#define rep(i,n) for(long long i=0; i<n; i++)
#define repr(i,n,m) for(long long i=m; i<n; i++)
#define END cout << endl
#define MOD 1000000007
#define pb push_back
#define sorti(x) sort(x.begin(), x.end())
#define sortd(x) sort(x.begin(), x.end(), std::greater<long long>())
#define debug(x) std::cerr << (x) << std::endl;
#define roll(x) for (auto&& itr : x) { debug(itr); }
 
template <class T> inline void chmax(T &ans, T t) { if (t > ans) ans = t;}
template <class T> inline void chmin(T &ans, T t) { if (t < ans) ans = t;}


// 最大公約数を求める(ユークリッドの互除法).
long long gcd (long long x, long long y) {
  if (y > x) swap(x,y);
  if (y == 0) return x;
  return gcd(x%y,y);
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);

  ll a,b;
  cin >> a >> b;

  if (gcd(a,b) != 1) {
    cout << -1 << endl;
  } else {
    cout << (a-1)*(b-1)/2<<endl;
  }
  
}