n,m = map(int,input().split())
maxi = n
if n%2 == 0:
mini = n//2
else:
mini = ((n-1)//2) + 1
if mini%m == 0:
steps = mini
else:
steps = mini + m - (mini%m)
if steps <= maxi:
print(steps)
else:
print(-1)
#include<bits/stdc++.h>
using namespace std;
int n,m,cnt=1e5+10;
int main()
{
cin >> n >> m;
if(n<m) puts("-1");
else
{
int res=n%2==0?n/2:(n+1)/2;
while(res%m) res=res-1+2;
cout << res << endl;
}
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