#include <fstream>
#include <vector>
using namespace std;
ifstream fin("orase.in");
ofstream fout("orase.out");
vector <int> v[1000005];
int a[50005];
int i,j,q,Max,n,m,x,y,k;
int main()
{
    fin>>n>>m;
    for(i=1;i<=m;i++)
    {
        fin>>x>>y;
        for(j=1;j<=k;j++)
            Max=max(Max,v[a[j]][0]+a[j]-x+y);
        if(v[a[j]].size()==0)
        {
            a[++k]=x;
            v[x].push_back(y);
        }
        else
        if(v[x][0]<y)
        v[x][0]=y;
    }
    fout<<Max;
    return 0;
}