#include <iostream>
#include<fstream>
#include<algorithm>

using namespace std;
    ifstream f("orase.in");
    ofstream g("orase.out");

    int n, m;
struct puncte{
    signed long d, in;}q[50010];

int cmp(puncte a, puncte b)
{  return a.d<b.d; }

int main()
{   int j,maxim=0, s=0;
    f>>n>>m;
for(int i=1; i<=n; i++)
    {   f>>q[i].d>>q[i].in; }
      sort(q+1, q+n+1, cmp);
   // for(int i=1; i<=n; i++) cout<<q[i].d<<" "<<q[i].in<<"\n";

    for(int i=0, j=1; j<=n; j++)
        {s=q[i].in+q[j].in+(q[j].d-q[i].d);
        if(s>maxim) maxim=s;
    if(q[j].in>q[i].in +(q[j].d-q[i].d)) i=j; }
    g<<maxim;
        return 0;
}