#include <iostream>
#include <fstream>
#include <cstdio>
#include <cmath>
#include <vector>
#include <cstring>
#include <string>
#include <set>
#include <map>
#include <stack>
#include <queue>
#include <deque>
#include <algorithm>
using namespace std;

#define REP(i,n) for(int i=0; i<n; ++i)
#define FOR(i,a,b) for(int i=a; i<=b; ++i)
#define FORR(i,a,b) for (int i=a; i>=b; --i)
#define ALL(c) (c).begin(), (c).end()

typedef long long ll;
typedef vector<int> VI;
typedef vector<ll> VL;
typedef vector<VL> VVL;
typedef vector<VI> VVI;
typedef pair<int,int> P;
typedef pair<ll,ll> PL;

int dp[101][2];

int main() {
    int n, d;
    cin >> n >> d;
    dp[0][1] = -1e9;
    REP(i,n){
        int x, y;
        cin >> x >> y;
        dp[i+1][0] = max(dp[i][0], dp[i][1] - d) + x;
        dp[i+1][1] = max(dp[i][1], dp[i][0] - d) + y;
    }
    cout << max(dp[n][0], dp[n][1]) << endl;

    return 0;
}