#include<bits/stdc++.h>
#define FILE "plant"
#define MAXN 100010
#define eps 1e-7
using namespace std;
typedef long long ll;
struct node{
	ll x,y; node(){}
	node(const ll a,const ll b):x(a),y(b){}
}p[MAXN],sta[MAXN];
int n,top;
ll d,a[MAXN],sum[MAXN];
double ans;
inline ll read(){
	ll x=0,f=1;  char ch=getchar();
	while(!isdigit(ch))  {if(ch=='-')  f=-1;  ch=getchar();}
	while(isdigit(ch))  {x=x*10+ch-'0';  ch=getchar();}
	return x*f;
}
double slop(node a,node b){return (double)(a.y-b.y)/(a.x-b.x);}
void insert(const node &b){
	while(top>=2&&slop(sta[top-1],sta[top])>=slop(sta[top],b)-eps) --top;
	sta[++top]=b;
}
double find(node b){
	int l=1,r=top;  double temp=0;
	while(l+3<=r){
		int mid1=(r-l)/3+l,mid2=(r-l)*2/3+l;
		if(slop(sta[mid1],b)>slop(sta[mid2],b))  r=mid2;
		else l=mid1;
	}
	for(int i=l;i<=r;++i)  temp=max(temp,slop(sta[i],b));
	return temp;
}
int main(){
	freopen(FILE".in","r",stdin);
	freopen(FILE".out","w",stdout);
	n=read();  d=read();
	for(int i=1;i<=n;++i){
		a[i]=read();  sum[i]=sum[i-1]+a[i];
		ll x=read();  insert(node(d*i,sum[i-1]));
		ans+=find(node(d*i+x,sum[i]));
	}
	printf("%.0lf\n",ans);
	return 0;
}