#include <iostream>
#include <vector>
#include <string>
#include <cstring>
#include <math.h>
#include <cmath>
#include <limits.h>
#include <map>
#include <set>
#include <queue>
#include <algorithm>
#include <functional>
#include <stdio.h>
using namespace std;

long long MOD = 1000000007;

int main() {
    
    int N;
    long long D;
    cin >> N >> D;
    long long a[2] = {0};
    a[1] = -D;

    for ( int i = 0; i < N; i++ ) {
        long long t,k;
        cin >> t >> k;
        long long x[2];
        x[0] = a[0]; x[1] = a[1];
        a[0] = max( x[0]+t, x[1]-D+t );
        a[1] = max( x[1]+k, x[0]-D+k );
    }
    cout << max( a[0], a[1] ) << endl;


    return 0;
}