#include <iostream>
#include <fstream>
#include <algorithm>
#define LL long long
using namespace std;

struct el{
    LL t,p;
};

LL N,C,dp[2010]; el a[2010];

inline bool cmp(el A, el B){
    return (A.t<B.t);
}

LL f(LL x, LL v){
    if (x<v) return 0;
    return v;
}

int main(){
    ifstream fin("carnati.in");
    ofstream fout("carnati.out");
    fin >> N >> C;

    int i,j; LL MAX=0,res=0;
    for (i=1; i<=N; i++) fin >> a[i].t >> a[i].p;
    sort(a+1,a+N+1,cmp);

    for (i=1; i<=N; i++){
        dp[1]=f(a[1].p,a[i].p)-C;
        MAX=dp[1];
        for (j=2; j<=N; j++){
            dp[j]=max(f(a[j].p,a[i].p)-C,dp[j-1]+f(a[j].p,a[i].p)-(a[j].t-a[j-1].t)*C);
            MAX=max(MAX,dp[j]);
        }
        res=max(MAX,res);
    }

    fout << res << "\n";
    return 0;
}