#include <cstdio>
#include <algorithm>

#define nmax 50010
using namespace std;

struct lol { int L, D; } A[nmax];
int i, n, m, sol, maxL;
inline bool cmp (const lol &a, const lol &b) {
	if (a.D < b.D) return 1;
	return 0;
}
int main () {
	freopen ("orase.in", "r", stdin);
	freopen ("orase.out", "w", stdout);
	scanf ("%d%d\n", &n, &m);
	for (i = 1; i <= n; i++)
		scanf ("%d%d\n", &A[i].D, &A[i].L);
	random_shuffle (A + 1, A + n + 1);
	sort (A + 1, A + n + 1, cmp);
	maxL = -nmax;
	for (i = 1; i <= n; i++) {
		if (sol < A[i].L + A[i].D + maxL) sol = A[i].L + A[i].D + maxL;
		if (maxL < A[i].L - A[i].D) maxL = A[i].L - A[i].D;
	}
	printf ("%d\n", sol);
	return 0;
}