#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;

typedef int _loop_int;
#define REP(i,n) for(_loop_int i=0;i<(_loop_int)(n);++i)
#define FOR(i,a,b) for(_loop_int i=(_loop_int)(a);i<(_loop_int)(b);++i)
#define FORR(i,a,b) for(_loop_int i=(_loop_int)(b)-1;i>=(_loop_int)(a);--i)

#define DEBUG(x) cout<<#x<<": "<<x<<endl
#define DEBUG_VEC(v) cout<<#v<<":";REP(i,v.size())cout<<" "<<v[i];cout<<endl
#define ALL(a) (a).begin(),(a).end()

#define CHMIN(a,b) a=min((a),(b))
#define CHMAX(a,b) a=max((a),(b))

// mod
const ll MOD = 1000000007ll;
#define FIX(a) ((a)%MOD+MOD)%MOD

// floating
typedef double Real;
const Real EPS = 1e-11;
#define EQ0(x) (abs(x)<EPS)
#define EQ(a,b) (abs(a-b)<EPS)
typedef complex<Real> P;

ll t;
int n;
ll x[125252],y[125252];
ll sum[125252];

int main(){
  scanf("%lld%d",&t,&n);
  REP(i,n)scanf("%lld%lld",x+i,y+i);
  REP(i,n)sum[i+1]=sum[i]+(y[i]-x[i]);
  ll ans = 0;
  {
    int ed = 0;
    REP(i,n){
      ll st = x[i];
      while(ed<n && st+t >= y[ed])ed++;
      CHMAX(ans,sum[ed]-sum[i]+(ed<n?max(0ll,st+t-x[ed]):0));
    }
  }
  // swap(x,y);
  // REP(i,n)x[i]=-x[i],y[i]=-y[i];
  // reverse(x,x+n);
  // reverse(y,y+n);
  // REP(i,n)sum[i+1]=sum[i]+(y[i]-x[i]);
  // {
  //   int ed = 0;
  //   REP(i,n){
  //     ll st = x[i];
  //     while(ed<n && st+t >= y[ed])ed++;
  //     CHMAX(ans,sum[ed]-sum[i]+(ed<n?max(0ll,st+t-x[ed]):0));
  //   }
  // }
  printf("%lld\n",ans);
  return 0;
}