#include<cstdio>
#include<cctype>
#include<iostream>
#include<fstream>
#include<functional>
#include<utility>
#include<vector>
#include<stack>
#include<map>
#include<queue>
#include<deque>
#include<cstdlib>
#include<cmath>
#include<algorithm>
#include<set>
#include<complex>
#include<cstring>
#include<string>
#include<cassert>
#include<iomanip>


using namespace std;

typedef long long LL;
typedef unsigned long long ULL;
typedef vector<int> vi;
typedef pair<int,int> pii;

#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define all(n) (n).begin(), (n).end()
#define EPS 1e-9
#define INF 1e9
#define forn(i, n) for(int i = 0; i < (n); ++i)
#define forab(i, a, b) for(int i = a; (i) < (b); ++(i))
#define forba(i, b, a) for(int i = b-1; i >= (a); --i)
#define forit(i, v) for(__typeof((v).begin()) i = (v).begin(); i != (v).end(); ++i)
#define fornr(i,n) for(int i=(n)-1;i>=0;--i)

void lift(int n, int m){
    if(n!=0 && m==1){
        return n;
    } else if(n==0){
        printf("-1");
        exit(0);
    } else {
        lift(n-1, (m-1)?():());
    }
}


int main(){
#ifdef FN
    freopen(FN".in", "r", stdin);
    freopen(FN".out", "w", stdout);
#endif
    int n, m;
    scanf("%d%d", &n, &m);
    lift(n, m);
    return 0;
}