#include <iostream>
#include <vector>
#include <string>
#include <map>
#include <algorithm>
#include <climits>
#include <cstdio>
#include <cstdlib>
#include <cctype>
#include <cassert>
#include <list>
using namespace std;

#define for if(0);else for

int solve(list<int>& blank)
{
  list<int>::iterator it1, it2;
  
  while (true) {
    const int size = blank.size();
    
    if (size == 0) { return -1; }
    if (size == 1) { return blank.front(); }
    it2 = it1 = blank.begin();
    ++it2; 
    int x = *it1;
    int y = *it2;
    //cout << "x=" << x << ", y=" << y << endl;
    if (x == y) { blank.pop_front(); continue; }
    
    if (x > y) {
      if(! (*it1 = *it1 % *it2))
        blank.pop_front();
    } else {
      if(! (*it2 = *it2 % *it1))
        blank.erase(it2);
    }
  }
  
}

int main()
{
  int N;
  cin >> N;
  list<int> blank;
  for (int i = 0; i < N; ++i) {
    int x; cin >> x;
    blank.push_back(x);
  }

  int ret = solve(blank);
  if (ret < 0) {
    cout << "IMPOSSIBLE" << endl;
  } else {
    cout << ret << endl;
  }
  return 0;
}