1 #include<cstdio>
 2 #include<queue>
 3 #include<iostream>
 4 #define N 100000
 5 #define LL long long
 6 #define mid ((l+r)>>1)
 7 using namespace std;
 8 const LL MAX=1e15;
 9 LL n,m,k,p,l,r;
10 LL h[N+5],a[N+5];
11 LL now[N+5];
12 typedef pair<LL,LL> P;
13 struct cmp{
14     bool operator()(P a,P b){
15         return a.first>b.first;//小顶堆
16     }
17 };
18 bool judge(LL x){
19     priority_queue<P,vector<P>,cmp> q;
20     for(LL i=0;i<n;++i){
21         now[i]=x;//刚开始竹子的高度为MAXi
22         if(x-m*a[i]>=0)continue;//不需要补充高度
23         q.push(make_pair(x/a[i],i));//P(到零的天数,下标)
24     }
25     LL times=0;//砍伐次数
26     for(;times<=k*m;times++){
27         if(q.empty())break;
28         P temp=q.top();
29         q.pop();
30         if(temp.first<=times/k)return 0;//来不及补充高度，竹子在k天前就长成负数
31         LL index=temp.second;
32         now[index]+=p;
33         if(now[index]-m*a[index]>=0)continue;//不需要补充高度
34         q.push(make_pair(now[index]/a[index],index));
35     }
36     if(times>k*m)return 0;
37     for(int i=0;i<n;++i){
38         LL temp=m*a[i]+h[i]-now[i];
39         if(temp<=0)continue;
40         else times+=((temp/p)+(LL)(temp%p!=0));//到h[i]还需要多少次补充高度
41         if(times>k*m)return 0;
42     }
43     return 1;
44 }
45 int main(void){
46     scanf("%I64d%I64d%I64d%I64d",&n,&m,&k,&p);
47     l=1,r=MAX;
48     for(LL i=0;i<n;++i)
49         scanf("%I64d%I64d",&h[i],&a[i]);
50     while(l<r){//二分
51         if(judge(mid))r=mid;
52         else l=mid+1;
53     }
54     printf("%I64d\n",l);
55 }