#define _USE_MATH_DEFINES
#include  <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#include <string>
#include<math.h>
#include<iomanip>
#include<stdio.h>
#include <stdlib.h>
#include<stdio.h>
#include <queue>
#include<map>
#include <fstream>
#include <sstream>
#include<set>
#include<stack>
#include<time.h>
#include <sys/types.h>
#include <tuple>
#include <numeric>
#include <iterator>

typedef long long int ll;
//#include<bits/stdc++.h>


using namespace std;

ll gcd(ll a, ll b) {
    if (a < b)swap(a, b);
    if (b == 0)return a;
    return gcd(b, a % b);
}
int lcm(int a, int b) {
    return a * b / gcd(a, b);
}







int main()
{

    int Q;
    Q = 1;
    //cin >> Q;   

    while (Q--) {
        int n;
        cin >> n;
        ll d;
        cin >> d;
        vector<ll>t(n+5), k(n+5);
        for (int i = 0; i < n; i++) {
            cin >> t[i] >> k[i];
        }
        vector<ll>x(n+5), y(n+5);
        x[0] = t[0]; y[0] = k[0] - d;
        for (int i = 0; i < n; i++) {
            x[i + 1] = max(x[i] + t[i + 1], y[i] + t[i + 1] - d);
            y[i + 1] = max(y[i] + k[i + 1], x[i] + k[i + 1] - d);
        }
        cout << max(x[n - 1], y[n - 1]);
    }
        return 0;
}