#include <fstream>
#include <algorithm>
#define int long long
using namespace std;
ifstream cin ("orase.in");
ofstream cout ("orase.out");
int d[100005], ind[100005], l[100005], cd[100005], cl[100005], maxx=0, minn=9999999, m;
int f(int a, int b) {
    return d[a]<d[b];
}
int32_t main()
{
    int n;
    cin >> m >> n;
    for(int i=1;i<=n;i++) {
        cin >> d[i] >> l[i];
        ind[i]=i;
    }
    sort(ind+1,ind+n+1,f);
    for(int i=1;i<=n;i++) {
        cd[i]=d[ind[i]];
        cl[i]=l[ind[i]];
    }
    for(int i=1;i<=n;i++) {
        maxx=max(maxx, cd[i]+cl[i]-minn);
        minn=min(minn,cd[i]-cl[i]);
    }
    cout << maxx;

    return 0;
}