#define _USE_MATH_DEFINES
#include <cstdio>
#include <iostream>
#include <sstream>
#include <fstream>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <complex>
#include <string>
#include <vector>
#include <list>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <bitset>
#include <numeric>
#include <limits>
#include <climits>
#include <cfloat>
#include <functional>
#include <iterator>
using namespace std;

int main()
{
    int n, d;
    cin >> n >> d;

    int a = 0;
    int b = INT_MIN / 2;
    for(int i=0; i<n; ++i){
        int t, k;
        cin >> t >> k;
        int a2 = max(a, b - d) + t;
        int b2 = max(b, a - d) + k;
        a = a2;
        b = b2;
    }
    cout << max(a, b) << endl;

    return 0;
}