//HDU 6119

#include <bits/stdc++.h>

using namespace std;

typedef long long LL;

const int mod = 1e9+7;

const int maxn = 1e5+10;

pair<LL,LL>p[maxn];

LL n,m,sum[maxn];

int main()

{

    while(~scanf("%lld%lld",&n,&m))

    {

        for(int i=1; i<=n; i++) scanf("%lld%lld",&p[i].first,&p[i].second);

        sort(p+1,p+n+1);

        int cnt=0;

        for(int i=1; i<=n; i++){

            LL l=i,r=i+1,last=p[i].second;

            while(r<=n&&p[r].first<=last){

                last=max(last,p[r].second),r++;

            }

            r--;

            p[cnt++]=make_pair(p[l].first,last);

            i=r;

        }

        sum[0]=0;

        for(int i=1; i<cnt; i++){

            sum[i]=sum[i-1]+p[i].first-p[i-1].second-1;

        }

        LL ans=m;

        int r=0;

        for(int l=0; r<cnt&&l<cnt; l++){

            while(sum[r]-sum[l]<=m&&r<cnt) r++;

            LL x = m-(sum[r-1]-sum[l]);

            ans = max(ans, p[r-1].second-p[l].first+1+x);

        }

        printf("%lld\n", ans);

    }

    return 0;

}