#include <bits/stdc++.h>

using namespace std;

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

    vector<int> T(n), K(n);
    for (int i = 0; i < n; ++i) {
        cin >> T[i] >> K[i];
    }

    int dp[101][2];
    dp[0][0] = T[0], dp[0][1] = K[0] - d;
    for (int i = 1; i < n; ++i) {
        dp[i][0] = max(dp[i - 1][0], dp[i - 1][1] - d) + T[i];
        dp[i][1] = max(dp[i - 1][0] - d, dp[i - 1][1]) + K[i];
    }

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