#include <iostream>
#include <algorithm>
#include <vector>
#include <map>
#include <string>
#include <numeric>
#include <cstring>
#include <cmath>
#include <set>
#include <climits>
#include <queue>
#include <stack>

const int MOD = 1e9 + 7;
const int iINF = 1000000000;
const long long int llINF = 1000000000000000000;
#define rep(i, n) for (int i = 0; i < (n); i++)

using namespace std;
using ll = long long int;
using P = pair<int, int>;
using edge = struct
{
    int to;
    int cost;
};

int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};

ll dp[100][2];

int main()
{
    int N, D;
    cin >> N >> D;
    int T, K;
    cin >> T >> K;

    dp[1][0] = T;
    dp[1][1] = K - D;

    for(int i = 1; i < N; i++)
    {
        cin >> T >> K;
        dp[i + 1][0] = max(dp[i][0] + T, dp[i][1] - D + T);
        dp[i + 1][1] = max(dp[i][1] + K, dp[i][0] - D + K);
    }

    cout << max(dp[N][0], dp[N][1]) << endl;
    return 0;
}