#include <bits/stdc++.h>
using namespace std;
ifstream f("orase.in");
ofstream g("orase.out");
struct cv{int d, l;};
cv v[1000001];
int main()
{
    int n,i,m,j,maxx=0;
    f>>n>>m;
    for(i=1;i<=m;++i)
            f>>v[i].d>>v[i].l;
    for(i=1;i<=m;++i)
        for(j=i+1;j<=m;++j)
            if(v[i].l+v[j].l+ abs(v[i].d-v[j].d)>maxx)
                maxx=v[i].l+v[j].l+ abs(v[i].d-v[j].d);
    g<<maxx;
    return 0;
}