#include<cstdio>

 #include<vector>

 #include<cstring>

 using namespace std;

 const int maxn = _____;

 struct TwoSAT

 {

     int n;

     vector<int> G[maxn*];

     bool mark[maxn*];

     int S[maxn*], c;

     bool dfs(int x)

     {

         if (mark[x^]) return false;

         if (mark[x]) return true;

         mark[x] = true;

         S[c++] = x;

         for (int i = ; i < G[x].size(); i++)

             if (!dfs(G[x][i])) return false;

         return true;

     }

     void init(int n) // 一定要注意初始化的点数，别弄错

     {

         this->n = n;

         for (int i = ; i < n*; i++) G[i].clear();

         memset(mark, , sizeof(mark));

     }

     // x = xval or y = yval

     void add_clause(int x, int xval, int y, int yval) // 编号从0~n-1

     {

         x = x *  + xval;

         y = y *  + yval;

         G[x^].push_back(y);

         G[y^].push_back(x);

     }

     bool solve()

     {

         for(int i = ; i < n*; i += )

             if(!mark[i] && !mark[i+])

             {

                 c = ;

                 if(!dfs(i))

                 {

                     while(c > ) mark[S[--c]] = false;

                     if(!dfs(i+)) return false;

                 }

             }

         return true;

     }

 };

 TwoSAT solver;

 ////////////////////////////////////////////////////

 //二分搜索，2-SAT中经常会用到

         int L=____,R=____;

         while(L < R)

         {

             int M = L + (R-L+)/;

             if(test(M)) L = M; // M满足条件的话test返回真

             else R = M-;

         }

         printf("%d\n",L);