#include <bits/stdc++.h>
typedef long long ll;
#define INF 1000000000
#define MOD 1000000007
using namespace std;
int main(void){
    int n,m,ans;
    cin>>n>>m;
    if(n<=m){
        ans=1;
    }else if(m==2){
        if(n==3){
            ans=-1;
        }else{
            ans=(n+1)/2;
        }
    }else{
        ans=n/m+(n-n/m*m)/m;
    }
    cout<<ans<<endl;
}