#include using namespace std; #define rep(i,n) for(ll i=0;i=0;i--) #define perl(i,r,l) for(ll i=r-1;i>=l;i--) #define fi first #define se second #define pb push_back #define ins insert #define pqueue(x) priority_queue,greater> #define all(x) (x).begin(),(x).end() #define CST(x) cout<; using vvl=vector>; using pl=pair; using vpl=vector; using vvpl=vector; const ll MOD=1000000007; const ll MOD9=998244353; const int inf=2e9+1; const ll INF=4e18; const ll dy[8]={-1,0,1,0,1,1,-1,-1}; const ll dx[8]={0,-1,0,1,1,-1,1,-1}; template inline bool chmax(T &a, T b) { return ((a < b) ? (a = b, true) : (false)); } template inline bool chmin(T &a, T b) { return ((a > b) ? (a = b, true) : (false)); } int main(){ ll a,b;cin >> a >> b; if(gcd(a,b)!=1)cout << -1 << endl,exit(0); set st; rep(i,200)rep(j,200)st.insert(a*i+b*j); ll ans=0; rep(i,a*b+1){ if(!st.count(i))ans++; } cout << ans << endl; }