#include <fstream>
#include <algorithm>

using namespace std;

ifstream cin("carnati.in");
ofstream cout("carnati.out");

const int MAXN = 2000;

pair<int, int> v[1 + MAXN];
int n, c;

int Try(int p) {
    int answer = 0, sum = 0;
    for (int i = 1; i <= n; i++) {
        int pay = (v[i].first - v[i - 1].first) * c;
        sum -= pay;
        sum = max(sum, 0);
        if (p <= v[i].second)
            sum += p;
        answer = max(answer, sum - c);
    }
    return answer;
}

int main() {
    cin >> n >> c;
    for (int i = 1; i <= n; i++)
        cin >> v[i].first >> v[i].second;
    sort(v + 1, v + n + 1);
    int answer = 0;
    for (int i = 1; i <= n; i++)
        answer = max(answer, Try(v[i].second));
    cout << answer << "\n";
    return 0;
}