#include <iostream>
#include <cstring>
#include <vector>
using namespace std;

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

    int income[110][2];
    for (int i = 0; i < N; i++) {
        cin >> income[i][0] >> income[i][1];
    }

    static int dp[110][2];
    memset(dp, -1, sizeof(dp));

    dp[0][0] = 0;
    for (int i = 0; i < N; i++) {
        for (int j = 0; j < 2; j++) {
            if (dp[i][j] < 0) continue;
            dp[i + 1][j] = max(dp[i + 1][j], dp[i][j] + income[i][j]);
            dp[i + 1][j ^ 1] = max(dp[i + 1][j ^ 1], dp[i][j] + income[i][j ^ 1] - D);
        }
    }
    int ans = max(dp[N][0], dp[N][1]);
    cout << ans << endl;

    return 0;
}