#include <stdio.h>

struct { int l, d; } o[50000];

int main() {
	int lung, nr, l, d, i, j;
	long long max=0, s;
	FILE *f=fopen ("orase.in", "r");
	FILE *g=fopen ("orase.out", "w");
	fscanf (f, "%d %d", &lung, &nr);
	for (i=0; i<nr; ++i)
		fscanf (f, "%d %d", &o[i].d, &o[i].l);
	for (i=0; i<nr; ++i)
		{
			l=o[i].l; d=o[i].d;
			for (j=i+1; j<nr; ++j)
				{
					s=l+o[j].l;
					if (d-o[j].d>0) s+=d-o[j].d;
					else s+=o[j].d-d;
					if (s>max) max=s;
				}
		}
	fprintf (g, "%lld\n", max);
	fclose(f);
	fclose(g);
	return 0;
}