#include <iostream>
#include<fstream>
#include<vector>
using namespace std;
const int tmax=1500;
vector<int> v[tmax+5];
int a[2005];
int n,el,c,i,j,t,sum,ssm,cnt,val;
int main()
{
    ifstream f("carnati.in");
    ofstream g("carnati.out");
    f>>n>>c;
    for(i=1;i<=n;i++)
    {
        f>>t>>a[i];
        v[t].push_back(a[i]);
    }
    for(cnt=1;cnt<=n;cnt++)
    {
        val=a[cnt];sum=0;
        for(i=0;i<=tmax;i++)
        {
            el=0;
            for(j=0;j<v[i].size();j++)
            {
                if(v[i][j]>=val)
                    el+=val;
            }
            el-=c;
            if(sum>0) sum+=el;
            else sum=el;
            if(sum>ssm) ssm=sum;
        }
    }
    g<<ssm;
    return 0;
}