A_Dreamoon_and_Stairs.cpp

#include<bits/stdc++.h>
using namespace std;

typedef  long long int          ll;
typedef  long double            ld;
typedef  string                 str;
typedef  vector<ll>             vll;
typedef  vector<string>         vs;
typedef  vector<pair<ll, ll>>   vpl;
typedef  set<ll>                sll;
typedef  map<ll,ll>             mll;
typedef  pair<int,int>          pint;
typedef  pair<ll,ll>            pll;
double   pi = acos(-1.0);
#define  debug(x)              cerr<<#x<< <<x<<endl;
#define  loop                   for(ll i=1; i<=n; i++)
#define  all(a)                 (a).begin(), (a).end()
#define  min3(a,b,c)            min(a,min(b,c))
#define  max3(a,b,c)            max(a,max(b,c))
#define  min4(a,b,c,d)          min(a,min(b,min(c,d)));
#define  max4(a,b,c,d)          max(a,max(b,max(c,d)));
#define  pb                     push_back;

const int N=1e5+10;
const int M=1e9+7;

int main()
{

    ios_base::sync_with_stdio(false);
    cin.tie(NULL);cout.tie(NULL);
    
    int n,m;
    cin>>n>>m;

    int l=ceil((double)n/2);
    int ans=-1;

    for(int i=l;i<=n;i++){
        if(i%m==0){
            ans=i;
            break;
        }
    }

    cout<<ans<<endl;

    return 0;
}