cf965C 二分+推方程

#include<bits/stdc++.h>
using namespace std;
#define ll long long
ll n,k,M,D,anss;
ll calc(ll x){
    ll tmp=(n/x-1)/k+1;
    return tmp;
}
int main(){
    cin>>n>>k>>M>>D;
    for(ll i=1;i<=D;i++){
        ll l=1,r=M,mid,ans=0;
        while(l<=r){
            mid=l+r>>1;
            ll t=calc(mid);
            if(t>=i) ans=mid,l=mid+1;
            else r=mid-1;
        }
        if(ans!=0)
            anss=max(anss,ans*i);
    }
    cout<<anss<<endl;
}