//why so pointless?
#include<bits/stdc++.h>
using namespace std;
#define N 200100
struct Event{
    int x;
    int ylo,yhi;
    bool type;//false = new
    friend bool operator<(const Event&a,const Event&b){
        return a.x<b.x;
    }
};
int n,m;
vector<pair<int,int>>poly;
vector<Event>ev;
struct Range{
    mutable int l;
    int r;
    friend bool operator<(const Range&x,const Range&y){
        return x.r<y.r;
    }
};
multiset<Range>end0,end1;
int cntVio0,cntVio1;
signed main(){
    iostream::sync_with_stdio(false);cin.tie(NULL);
    cin>>n>>m;
    for(int a=0;a<n;++a){
        int x,y;cin>>x>>y;
        poly.push_back({x,y});
    }
    bool signOpen=false;
    for(int i=0;i<poly.size();++i){
        int j=(i+1==poly.size()?0:i+1);
        if(poly[i].first!=poly[j].first)continue;
        if(poly[i].second<poly[j].second)ev.push_back({poly[i].first,poly[i].second,poly[j].second,false});
        else ev.push_back({poly[i].first,poly[j].second,poly[i].second,true});
        if(poly[i].first==0)signOpen=ev.back().type;
    }
    for(auto&a:ev)a.type^=signOpen;
    sort(ev.begin(),ev.end());
    int ans=0;
    for(int leftEvent=0;leftEvent<ev.size()&&ev[leftEvent].x!=m;){
        if(cntVio0)break;
        if(end1.empty())ans=max(ans,ev[leftEvent].x);
        int rightEvent=leftEvent;
        while(ev[leftEvent].x==ev[rightEvent].x)++rightEvent;
        for(int i=leftEvent;i<rightEvent;++i){
            Event e=ev[i];
            if(!e.type){//ins
                auto it=end0.insert({e.ylo,e.yhi});
                cntVio0+=(it->l^it->r)&1;
                //for(auto x:end0)cout<<"("<<x.l<<","<<x.r<<")";cout<<endl;
                if(it!=end0.begin()&&prev(it)->r==it->l){
                    cntVio0+=((it->r^prev(it)->l)&1)-((it->r^it->l)&1)-((prev(it)->r^prev(it)->l)&1);
                    it->l=prev(it)->l;
                    end0.erase(prev(it));
                }
                if(next(it)!=end0.end()&&it->r==next(it)->l){
                    ++it;
                    cntVio0+=((it->r^prev(it)->l)&1)-((it->r^it->l)&1)-((prev(it)->r^prev(it)->l)&1);
                    it->l=prev(it)->l;
                    end0.erase(prev(it));
                }
            }
            else{//del
                auto it=end0.lower_bound({e.ylo,e.yhi});
                if(!(it->l<=e.ylo&&e.yhi<=it->r))break;
                cntVio0+=((e.ylo^it->l)&1)+((e.yhi^it->r)&1)-((it->l^it->r)&1);
                if(it->l<e.ylo)end0.insert({it->l,e.ylo});
                if(e.yhi<it->r)end0.insert({e.yhi,it->r});
                end0.erase(it);
            }
        }
        end0.swap(end1);
        swap(cntVio0,cntVio1);
        if(ev[rightEvent].x>ev[leftEvent].x+1){
            if(cntVio0)break;
            if(end1.empty())ans=max(ans,ev[leftEvent].x+1);
            end0.swap(end1);
            swap(cntVio0,cntVio1);
            if(ev[rightEvent].x>ev[leftEvent].x+2){
                if(cntVio0)break;
                if(end1.empty())ans=max(ans,ev[rightEvent].x-((ev[rightEvent].x^ev[leftEvent].x)&1));
                if(end0.empty())ans=max(ans,ev[rightEvent].x-((ev[rightEvent].x^ev[leftEvent].x^1)&1));
                if((ev[rightEvent].x^ev[leftEvent].x)&1){
                    end0.swap(end1);
                    swap(cntVio0,cntVio1);
                }
            }
        }
        leftEvent=rightEvent;
    }
    cout<<ans<<endl;
}