#include<fstream>
#include<algorithm>
using namespace std;
ifstream f("orase.in");
ofstream g("orase.out");
int n,m,i,d,rez;
struct oras{
	int x,y;
};
oras v[50001];
bool cmp(oras a,oras b)
{if(a.x<b.x)
	return 1;
return 0;
}
int main()
{f>>m>>n;
for(i=1;i<=n;++i)
	f>>v[i].x>>v[i].y;
sort(v+1,v+n+1,cmp);
d=v[1].y;
for(i=2;i<=n;++i)
{d=d+v[i].x-v[i-1].x;
rez=max(rez,d+v[i].y);
d=max(d,v[i].y);
}
g<<rez<<'\n';
return 0;
}