#include <cstdio>
#include <algorithm>

#define NMax 2005
#define T first
#define P second

using namespace std;

int N, C, DP[NMax], S;
pair <int, int> Client[NMax];

inline void SolveDP (int CurrentP)
{
    for (int i=1; i<=N; ++i)
    {
        int Sold=0;
        if (Client[i].P>=CurrentP)
        {
            Sold=CurrentP;
        }
        DP[i]=max (Sold-C, DP[i-1]+Sold-(Client[i].T-Client[i-1].T)*C);
        S=max (S, DP[i]);
    }
}

void Solve ()
{
    sort (Client+1, Client+N+1);
    for (int i=1; i<=N; ++i)
    {
        SolveDP (Client[i].P);
    }
}

void Read ()
{
    freopen ("carnati.in", "r", stdin);
    scanf ("%d %d", &N, &C);
    for (int i=1; i<=N; ++i)
    {
        scanf ("%d %d", &Client[i].T, &Client[i].P);
    }
}

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

int main()
{
    Read ();
    Solve ();
    Print ();
    return 0;
}