#include <fstream>

using namespace std;

ifstream in("carnati.in");
ofstream out("carnati.out");

struct client
{
                int timp,bani;
}v[2001];

int main()
{
    long long n,sg,sum=0,inc=1,profitulluigigel=-1,profitmax=-1,t,pmax=-1;
    in>>n>>sg;
    for(int i=1;i<=n;i++)
                in>>v[i].timp>>v[i].bani;
    for(int i=1;i<=n;i++)
    {
                sum=0;
                inc=1;
                for(int j=1;j<=n;j++)
                {
                                if(sum<t*sg)
                                {
                                                sum=0;
                                                inc=j;
                                }
                                if(v[j].bani>=v[i].bani)
                                                sum+=v[i].bani;
                                t=v[j].timp-v[inc].timp+1;
                                profitulluigigel=sum-t*sg;
                                if(profitulluigigel>profitmax)
                                                profitmax=profitulluigigel;
                }
    }
    out<<profitmax;
    return 0;
}