#include <fstream>
#include <algorithm>
#define ll long long
using namespace std;

const int NMAX = 50004;

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

struct hell
{
    int d, l;
}v[NMAX];
bool cmp(hell a, hell b)
{
    if (a.l != b.l)
        return a.l > b.l;
    return a.d > b.d;
}
int main()
{
    int n, m, i, ans = 0;
    cin >> n >> m;
    for (i = 1; i <= n; i++)
        cin >> v[i].d >> v[i].l;
    sort(v + 1, v + n + 1, cmp);
    for (i = 1; i + 1 <= n; i++)
        ans = max(ans, v[i].l + v[i + 1].l + abs(v[i].d - v[i + 1].d));
    cout << ans;
}