#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <vector>
using namespace std;

#define ABS(x) ((x) >= 0 ? (x) : -(x))
#define MP make_pair
#define REP(i, N) for (int i = 0; i < (int)(N); ++i)

pair<int, int> a[50005];
int M, N, x, y, mx = 0, s;

int main(void)
{
	freopen("orase.in", "r", stdin);
	freopen("orase.out", "w", stdout);
	scanf("%d %d", &M, &N);
	REP(i, N)
	{
		scanf("%d %d", &x, &y);
		a[i] = MP(x, y);
		if (x+y > mx) mx = x+y, s = i;
	}
	mx = 0;
	REP(i, N)
		mx >?= a[i].second + ABS(a[i].first - a[s].first) + a[s].second;
	printf("%d\n", mx);
	return 0;
}