# include <bits/stdc++.h>
# define x first
# define y second
using namespace std;
ifstream fi("orase.in");
ofstream fo("orase.out");
pair < int , int > s[50005];
int main(void)
{
    int m,n;
    fi>>m>>n;
    int p=0,u=0;
    for (int i=1;i<=n;++i) fi>>s[i].x>>s[i].y;
    int Max=0;
    sort(s+1,s+1+n);
    p=s[1].x,u=s[1].y;
    for (int i=2;i<=n;++i)
    {
        Max=max(Max,u+s[i].x-p+s[i].y);
        if (u+s[i].x-p < s[i].y) p=s[i].x,u=s[i].y;
    }
    return fo << Max << '\n',0;
}