https://ac.nowcoder.com/acm/contest/332/B

C++版本一

题解：

二分

 

/*
*@Author:   STZG
*@Language: C++
*/
#include <bits/stdc++.h>
#include<iostream>
#include<algorithm>
#include<cstdlib>
#include<cstring>
#include<cstdio>
#include<string>
#include<vector>
#include<bitset>
#include<queue>
#include<deque>
#include<stack>
#include<cmath>
#include<list>
#include<map>
#include<set>
//#define DEBUG
#define RI register int
using namespace std;
typedef long long ll;
//typedef __int128 lll;
const int N=100000+10;
const int MOD=1e9+7;
const double PI = acos(-1.0);
const double EXP = 1E-8;
const int INF = 0x3f3f3f3f;
ll t,n,m,k,q;
ll ans,cnt,flag,temp;
int a[N];
char str;
int main()
{
#ifdef DEBUG
	freopen("input.in", "r", stdin);
	//freopen("output.out", "w", stdout);
#endif
    scanf("%lld%lld%lld%lld",&n,&m,&k,&q);
    //scanf("%d",&t);
    ll l=1;
    ll r=q;
    while(l<=r){
        ll mid=(l+r)>>1;
        temp=mid*n+mid*(mid-1)/2*k;
        //cout<<mid<<endl;
        if(temp>=m){
            ans=mid;
            r=mid-1;
        }else{
            l=mid+1;
        }
    }
    cout << ans << endl;
    //cout << "Hello world!" << endl;
    return 0;
}