#include <bits/stdc++.h>
#define pp pair<int, int>
#define ll long long

using namespace std;

const int mxn = 50 * 1000 + 10;

ll l_total, m;

pp v[ mxn ];

ll cel_mare;

int main()
{
    ifstream cin("orase.in");
    ofstream cout("orase.out");
    cin>> l_total >> m;
    for(int i = 0, x, y; i < m; i++){
        cin>> x >> y;
        v[ i ] = make_pair(x, y);
    }
    sort(v, v + m);
    ll mx = v[ 0 ].second;
    for(int i = 1; i < m; i++){
        mx += v[ i ].first - v[ i - 1 ].first;
        cel_mare = max(cel_mare, mx + v[ i ].second);
        mx = max(mx, (long long) v[ i ].second);
    }
    cout<< cel_mare;
    return 0;
}