#include <bits/stdc++.h>
using namespace std;
const int NMAX=50000;
struct Oras {
    int D,L;
};
Oras v[NMAX+1];
bool cmp(const Oras &a, const Oras &b) {
    return a.D<b.D;
}
int main() {
    ifstream fin("secv2.in");
    ofstream fout("secv2.out");
    int m,n;
    fin>>m>>n;
    for(int i=1;i<=n;i++)
        fin>>v[i].D>>v[i].L;
    sort(v+1,v+n+1,cmp);
    long long mindif=(long long)v[1].D-v[1].L;
    long long r=0;
    for(int i=2;i<=n;i++){
        long long cand=(long long)v[i].D+v[i].L-mindif;
        if(cand>r)
            r=cand;
        if((long long)v[i].D-v[i].L<mindif)
            mindif=(long long)v[i].D-v[i].L;
    }
    fout<<r<<"\n";
    return 0;
}