# include <cstdio>

using namespace std;

int m, n, sum, s, i, j, x, d[50005], l[50005];
int main()
{
    freopen("orase.in","r",stdin);
    freopen("orase.out","w",stdout);
    
    scanf("%d%d",&m,&n);
    for (i = 1; i <= n; i++)
        scanf("%d%d",&d[i],&l[i]);
        
    for (i = 1; i <= n; i++)
        for (j = i + 1; j <= n; j++)
        {
            s = l[i] + l[j] + ((d[i] - d[j]) > 0 ? d[i] - d[j] : d[j] - d[i]);
            if (sum < s) sum = s;
            }
            
    printf("%d",sum);
    
return 0;
}