#include <iostream>
#include <fstream>
using namespace std;
long G,W,x,y,i,z,t,tt;
int main()
{fstream fin,fout;
  fin.open("energii.in",ios::in);
  fout.open("energii.out",ios::out);
  fin>>G;fin>>W;t=0;
  for (i=1;i<G;i++)
  {
      fin>>x;
      fin>>y;z=y;
      if (x>=W || y<=z)z=y;
      else if (x+t>W || x+tt<z)z=x+tt;
             else {t=x;tt=y;};
  };fout<<z;
  fin.close();
  fout.close();
    return 0;
}