#include <cstdio>
#include <algorithm>
using namespace std;

struct sp
{
    int d,l;
} v[50010];
int n;

bool cmp(sp a,sp b)
{
    if(a.d==b.d)
        return a.l<b.l;
    return a.d<b.d;
}

int m;
int c[1000010];
int d[1000010];

int sol;

int main()
{
    freopen("orase.in","r",stdin);
    freopen("orase.out","w",stdout);
    scanf("%d%d",&m,&n);

    for(int i=1;i<=n;i++)
        scanf("%d%d",&v[i].d,&v[i].l);
    sort(v+1,v+n+1,cmp);
    for(int i=1;i<=n;i++)
    {
        d[v[i].d]=v[i].l;
        if(v[i].d==v[i-1].d)
            sol=max(sol,v[i].l+v[i-1].l);
    }

    for(int i=1;i<=m;i++)
    {
        c[i]=max(c[i-1],d[i-1])+1;
        if(d[i])
            sol=max(sol,c[i]+d[i]);
    }
    printf("%d",sol);
    return 0;
}