#include <bits/stdc++.h>
#define inf 0x3f3f3f3f
#define met(a,b) memset(a,b,sizeof a)
#define pb push_back
#define mp make_pair
#define rep(i,l,r) for(int i=(l);i<=(r);++i)
#define inf 0x3f3f3f3f
using namespace std;
typedef long long ll;
const int N = 1e5+50;;
const int M = 17;
const int mod = 1e9+7;
const int mo=123;
const double pi= acos(-1.0);
typedef pair<int,int>pii;
int n,m;
ll sum[N],sum2[N];
vector<pii>vec;
struct man{
   int l,r;
}a[N];
bool cmp(const man &s,const man &d){
    if(s.l==d.l)return s.r<d.r;
    return s.l<d.l;
}
int main(){
    while(~scanf("%d%d", &n, &m)) {
        for(int i=1;i<=n;i++){
            scanf("%d%d",&a[i].l,&a[i].r);
        }
        sort(a+1,a+1+n,cmp);
        int now=1;
        vec.clear();
        met(sum,0);met(sum2,0);
        for(int i=2;i<=n;i++){
            if(a[i].l<=a[now].r)a[now].r=max(a[now].r,a[i].r);
            else {
                vec.pb(mp(a[now].l,a[now].r));
                now=i;
            }
        }
        vec.pb(mp(a[now].l,a[now].r));
        int sz=vec.size();
        ll ans=0,ss,dd;
        sum[0]=0;
        sum2[0]=vec[0].second-vec[0].first+1;
        for(int i=1;i<sz;i++){
            sum[i]=sum[i-1]+vec[i].first-vec[i-1].second-1;
            sum2[i]=sum2[i-1]+vec[i].second-vec[i].first+1;
        }
        for(int i=0;i<sz;i++){
            int ll=0,rr=i,res=-1;
            while(ll<=rr){
                int mid=(ll+rr)/2;
                ss=sum[i]-sum[mid];
                if(ss<=m)rr=mid-1,res=mid;
                else ll=mid+1;
            }
            if(res==0)dd=sum2[i];
            else dd=sum2[i]-sum2[res-1];
            ss=sum[i]-sum[res];
            ans=max(ans,dd+ss+(m-ss));
        }
        printf("%lld\n",ans);
    }
    return 0;
}