#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;
const int INF = (int)1e9 + 7;

int main() {
    int n, d; cin >> n >> d;
    vector<long long> dp{0, -d};
    while (n--) {
        int t, k; cin >> t >> k;
        vector<long long> dp2(2);
        dp2[0] = max(dp[0] + t, dp[1] - d + t);
        dp2[1] = max(dp[1] + k, dp[0] - d + k);
        swap(dp, dp2);
    }
    cout << max(dp[0], dp[1]) << endl;
    return 0;
}