#include<bits/stdc++.h>
using namespace std;
pair<int,int> v[50005],v1[50005];
bool comp(pair<int,int> a,pair<int,int> b)
{
   int  x1=a.second-a.first;
   int x2=b.second-b.first;
    if(x1<=x2) return 1;
        return 0;
}
inline int max(int a,int b)
{
    return a>b?a:b;
}
int n,m,val,maxim;
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].first,&v[i].second);
        v1[i]=make_pair(v[i].first,v[i].second);
    }
    sort(v+1,v+n+1,comp);
    for(int i=1;i<=n;i++)
    {
        val=v1[i].first+v1[i].second+v[n].second-v[n].first;
        maxim=max(maxim,val);
    }
    printf("%d\n",maxim);
    return 0;
}