using namespace std;
#include<cstdio>
#include<algorithm>
#define Nm 2048
#define f first
#define s second
#define max(a,b) ((a)>(b)?(a):(b))
int L[Nm],R[Nm],n,c,ans;
pair<int,int> A[Nm],B[Nm];

void read()
{
    int i;

    freopen("carnati.in","r",stdin);
    scanf("%d%d",&n,&c);
    for(i=0;i<n;++i)
        scanf("%d%d",&A[i].f,&A[i].s);
}

void solve()
{
    int i,j,m;

    sort(A,A+n);
    for(i=0;i<n;++i)
    {
        for(m=j=0;j<n;++j)
            if(A[i].s<=A[j].s)
                B[++m]=A[j];
        L[1]=0;
        for(j=2;j<=m;++j)
            L[j]=max(0,L[j-1]+A[i].s-(B[j].f-B[j-1].f)*c);
        R[m]=0;
        for(j=m-1;j;--j)
            R[j]=max(0,R[j+1]+A[i].s-(B[j+1].f-B[j].f)*c);
        for(j=1;j<=m;++j)
            ans=max(ans,L[j]+R[j]-c+A[i].s);
    }
}

void write()
{
    freopen("carnati.out","w",stdout);
    printf("%d\n",ans);
}

int main()
{
    read();
    solve();
    write();
    return 0;
}