#include <stdio.h>
#include <algorithm>
#define N 50000
struct cord{int dist;int lung;} v[N];
bool cmp(cord a, cord b)
{
    if(a.dist<b.dist)
        return true;
    return false;
}
int main()
{
    FILE *fin,*fout;
    fin=fopen("orase.in","r");
    fout=fopen("orase.out","w");
    int n,m;
    fscanf(fin,"%d%d",&m,&n);
    int i;
    for(i=0;i<n;i++)
        fscanf(fin,"%d%d",&v[i].dist,&v[i].lung);
    int j,max=0;
    std::sort(v,v+n,cmp);
    i=1;
    max=v[0].lung+v[0].dist;
    for(j=2;j<n;j++)
    {
        int temp=v[i].lung+v[j].lung+v[j].dist-v[i].dist;
        if(v[j].lung>v[i].lung+v[j].dist-v[i].dist)
            i=j;
        if(temp>max)
            max=temp;
    }
    fprintf(fout,"%d",max);
    return 0;
}