#include <bits/stdc++.h>

#define LL long long
#define VI vector<int>
#define VVI vector<vector<int>>
#define VVVI vector<vector<vector<int>>>
#define VB vector<bool>
#define VL vector<long long>
#define FOR(i,a,b) for(int i= (a); i<((int)b); ++i)
#define RFOR(i,a) for(int i=(a); i >= 0; --i)
#define FOE(i,a) for(auto i : a)
#define ALL(c) (c).begin(), (c).end()
#define RALL(c) (c).rbegin(), (c).rend()
#define DUMP(x)  cerr << #x << " = " << (x) << endl;
#define SUM(x) std::accumulate(ALL(x), 0LL)
#define MIN(v) *std::min_element(v.begin(), v.end())
#define MAX(v) *std::max_element(v.begin(), v.end())
#define EXIST(v,x) (std::find(v.begin(), v.end(), x) != v.end())
#define BIT(n) (1LL<<(n))
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end());
#define EPS 1e-14

const std::string YES = "YES";
const std::string Yes = "Yes";
const std::string NO = "NO";
const std::string No = "No";

bool inside(int y, int x, int H, int W) { return 0 <= y && y < H && 0 <= x && x < W; }

// 4近傍（右, 下, 左, 上）
const std::vector<int> dy = { 0, -1, 0, 1 };
const std::vector<int> dx = { 1, 0, -1, 0 };

using namespace std;

int main() {
    int N, D;
    cin >> N >> D;
    vector<int> T(N), K(N);
    FOR(i, 0, N) {
        cin >> T[i] >> K[i];
    }

    vector<vector<LL>> dp(N + 1, vector<LL>(2, -1));
    dp[0][0] = 0;
    dp[0][1] = -D;

    FOR(i, 0, N) {
        int t = T[i];
        int k = K[i];

        // tokyo
        dp[i + 1][0] = max(dp[i + 1][0], dp[i][0] + t);
        dp[i + 1][0] = max(dp[i + 1][0], dp[i][1] + t  - D);

        // kyoto
        dp[i + 1][1] = max(dp[i + 1][1], dp[i][1] + k);
        dp[i + 1][1] = max(dp[i + 1][1], dp[i][0] + k  - D);

    }

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


    return 0;
}