#include<bits/stdc++.h>
#define ll long long
using namespace std;
int a[1000001];
bool b;
ll n, k, um=0, mx;
int main()
{
	cin>>n>>k;
	memset(a, 0, sizeof(a));
	for (int i=0;i<n;i++)
	{
		ll x,y;
		cin>>x>>y;
		a[x]+=y;
		mx=max(x,mx);
	}
	for (ll i=k; i<=mx; i++)
	{
		if(a[i]>0 && a[i-k]>0){
			if(k==0){
				if(a[i]==1) continue;
				um = max(um,a[i]*i), b=1;
			}
			else
				um = max(um, min(a[i],a[i-k])*(i+i-k)),b=1;
		}
	}
	if (b==0)
		cout<<"NO";
	else	
		cout<<um;
    return 0;
}

为什么上面的

if(a[i]>0 && a[i-k]>0)

换成

if(a[i]!=0 && a[i-k]!=0)

才能AC ，哭了