添加链接描述
先模拟前（n-m）/（m-1）*2

#include<bits/stdc++.h>
using namespace std;
const int N=1100;
#define int long long
typedef long long ll;
signed main(){
    int T;
    cin>>T;
    while(T--){
        int n,m;
        ll res=0;
        cin>>n>>m;
        if(m==1&&n!=1){
            puts("-1");
            continue;
        }
        else if(m==1&&n==1){
            puts("1");
            continue;
        }
        if(n<=m){
            puts("1");
        }
        else {
            
            ll p=(n-m)/(m-1);
            ll res=p*2;
            ll now=p*(m-1);
            n-=now;
            while(n>0){
                if(n<=m){
                    res++;
                    break;
                }
                n-=m-1;
                res+=2;
            }
            // cout<<p<<endl;
        cout<<res<<endl;
        }
        
    }
    
    return 0;
}