#include <algorithm>
#include <iostream>
#include <cstdio>
using namespace std;
const int maxn = 2005;
pair <int, int> v[maxn];
int n, c;
int main()
{
    freopen("carnati.in", "r", stdin);
    freopen("carnati.out", "w", stdout);
    scanf("%d%d", &n, &c);
    for(int i = 1; i <= n; i++)
        scanf("%d%d", &v[i].first, &v[i].second);
    sort(v + 1, v + n + 1);
    int mx = 0;
    for(int i = 1; i <= n; i++)
    {
        int s = 0;
        for(int j = 1; j <= n; j++)
        {
            int salariu = (v[j].first - v[j-1].first) * c;
            s = s - salariu;
            s = max(s, 0);
            if(v[i].second <= v[j].second)
                s += v[i].second;
            mx = max(mx, s - c);
        }
    }
    printf("%d\n", mx);
    return 0;
}