#include <bits/stdc++.h>
#define ll long long
#define MOD 666013
using namespace std;
int N, M;
struct drum{
    int p, l;
};
drum A[50006];
bool comp(drum a, drum b){
    return a.p < b.p;
}
int rs1;
int main(){
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    ifstream cin("orase.in");
    ofstream cout("orase.out");
    cin >> M >> N;
    for(int i = 1; i <= N; i++)
        cin >> A[i].p >> A[i].l;
    sort(A+1, A+N+1, comp);
    int j = 1;
    for(int i = 2; i <= N; i++){
        rs1 = max(A[j].l - A[j].p + A[i].l + A[i].p, rs1);
    if(A[i].l - A[i].p > A[j].l - A[j].p)
        j = i;
    }
    cout << rs1;
    return 0;
}