#include <cstdio>
#include <algorithm>
using namespace std ;
const int DMAX = 50005 ;
struct oras
{
    int d,l;
}O[DMAX];
bool comp(oras a, oras b)
{
    return a.d < b.d;
}
int main ()
{
    freopen("orase.in" , "r"  , stdin); freopen("orase.out" , "w" , stdout);
    int N, m, i, mx = -2e9,S = -2e9;
    scanf("%d%d", &m, &N);
    for(i = 1 ; i <= N ; ++i)
        scanf("%d%d", &O[i].d, &O[i].l);
    sort(O + 1,O + N + 1,comp);
    for(i = 1;i <= N;++i)
    {
        if(S < mx + O[i].d + O[i].l)
            S = mx + O[i].d + O[i].l;
        if(mx < O[i].l - O[i].d)
            mx = O[i].l - O[i].d;
    }
    printf("%d", S);

}