题目传送门

　　传送门I

　　传送门II

　　传送门III

题目大意

　　有$n​$门科目有考试，第$i​$门科目有两场考试，时间分别在$a_i, b_i\ \ (a_i < b_i)​$，要求每门科目至少参加一场考试，不能在同一个时间参加两场考试，问最后参加的考试最早的时间是什么。

 /**

  * Codeforces

  * Problem#1027F

  * Accepted

  * Time: 873ms

  * Memory: 63000k

  */

 #include <algorithm>

 #include <iostream>

 #include <cstdlib>

 #include <cstdio>

 #include <vector>

 using namespace std;

 typedef bool boolean;

 template <typename T>

 void pfill(T* pst, const T* ped, T val) {

     for ( ; pst != ped; *(pst++) = val);

 }

 typedef class Input {

     protected:

         const static int limit = ;

         FILE* file; 

         int ss, st;

         char buf[limit];

     public:

         Input():file(NULL)    {    };

         Input(FILE* file):file(file) {    }

         void open(FILE *file) {

             this->file = file;

         }

         void open(const char* filename) {

             file = fopen(filename, "r");

         }

         char pick() {

             if (ss == st)

                 st = fread(buf, , limit, file), ss = ;//, cerr << "str: " << buf << "ed " << st << endl;

             return buf[ss++];

         }

 }Input;

 #define digit(_x) ((_x) >= '0' && (_x) <= '9')

 Input& operator >> (Input& in, unsigned& u) {

     char x;

     while (~(x = in.pick()) && !digit(x));

     for (u = x - ''; ~(x = in.pick()) && digit(x); u = u *  + x - '');

     return in;

 }

 Input& operator >> (Input& in, unsigned long long& u) {

     char x;

     while (~(x = in.pick()) && !digit(x));

     for (u = x - ''; ~(x = in.pick()) && digit(x); u = u *  + x - '');

     return in;

 }

 Input& operator >> (Input& in, int& u) {

     char x;

     while (~(x = in.pick()) && !digit(x) && x != '-');

     int aflag = ((x == '-') ? (x = in.pick(), -) : ());

     for (u = x - ''; ~(x = in.pick()) && digit(x); u = u *  + x - '');

     u *= aflag;

     return in;

 }

 Input& operator >> (Input& in, long long& u) {

     char x;

     while (~(x = in.pick()) && !digit(x) && x != '-');

     int aflag = ((x == '-') ? (x = in.pick(), -) : ());

     for (u = x - ''; ~(x = in.pick()) && digit(x); u = u *  + x - '');

     u *= aflag;

     return in;

 }

 Input in (stdin);

 #define pii pair<int, int>

 int n, m;

 int *uf;

 int *es;

 int *mx, *smx;

 pii* ps;

 vector<int> br;

 int find(int x) {

     return (uf[x] == x) ? (x) : (uf[x] = find(uf[x]));

 }

 inline void init() {

     in >> n;

     ps = new pii[(n + )];

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

         in >> ps[i].first >> ps[i].second;

 }

 inline void descrete() {

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

         br.push_back(ps[i].first);

         br.push_back(ps[i].second);

     }

     sort(br.begin(), br.end());

     m = unique(br.begin(), br.end()) - br.begin();

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

         ps[i].first = lower_bound(br.begin(), br.begin() + m, ps[i].first) - br.begin();

         ps[i].second = lower_bound(br.begin(), br.begin() + m, ps[i].second) - br.begin();

     }

 }

 void addEdge(int u, int v) {

     if (find(u) != find(v)) {

         u = find(u), v = find(v);

         es[u] += es[v];

         if (mx[u] < mx[v])

             smx[u] = mx[u], mx[u] = mx[v];

         else if (smx[u] < mx[v])

             smx[u] = mx[v];

         smx[u] = ((smx[u] > smx[v]) ? (smx[u]) : (smx[v]));

         uf[v] = u;

     }

     es[find(u)]++;

 }

 int res = -;

 inline void solve() {

     uf = new int[m];

     es = new int[m];

     mx = new int[m];

     smx = new int[m];

     for (int i = ; i < m; i++)

         uf[i] = i, es[i] = -, mx[i] = i, smx[i] = -;

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

         addEdge(ps[i].first, ps[i].second);

     for (int i = ; i < m; i++)

         if (find(i) == i) {

             if (es[i] > ) {

                 res = -;

                 break;

             }

             if (!es[i])

                 res = max(res, mx[i]);

             else

                 res = max(res, smx[i]);

         }

     if (res == -)

         puts("-1");

     else

         printf("%d\n", br[res]);

 }

 int main() {

     init();

     descrete();

     solve();

     return ;

 }