#include using namespace std; #define F first #define S second #define pi M_PI #define R cin>> #define Z class #define ll long long #define ln cout<<'\n' #define in(a) insert(a) #define pb(a) push_back(a) #define pd(a) printf("%.10f\n",a) #define mem(a) memset(a,0,sizeof(a)) #define all(c) (c).begin(),(c).end() #define iter(c) __typeof((c).begin()) #define rrep(i,n) for(int i=(int)(n)-1;i>=0;i--) #define REP(i,m,n) for(int i=(int)(m);i<(int)(n);i++) #define rep(i,n) REP(i,0,n) #define tr(it,c) for(iter(c) it=(c).begin();it!=(c).end();it++) templatevoid pr(A a){cout<void pr(A a,B b){cout<void pr(A a,B b,C c){cout<void pr(A a,B b,C c,D d){cout<void PR(A a,ll n){rep(i,n){if(i)cout<<' ';cout<=0&&x=0&&y P; void Main() { ll n,m; cin >> n >> m; if(n<=m) pr(1); else if(n%2==0&&n/m<=2) pr(ceil((double)n/m)); else pr(-1); } int main(){ios::sync_with_stdio(0);cin.tie(0);Main();return 0;}