#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  cout << std::setprecision(8);
  long double a[1010];
  long double mi = 1000000000;
  long double ma = 1;
  for (int i = 1; i <= n; i++) {
    cin >> a[i];
  }
  if (n == 1) {
    cout << "not unique";
    return 0;
  }
  a[0] = 0;
  for (int i = 0; i <= n - 1; i++) {
    long double d = a[i] * 10;
    long double di = (double)(i + 1);
    long double d2 = a[i + 1] - a[i];
    d2 = d2 * 10.0;
    long double d3 = d2 + 10.0;
    long double an = (d2 + d) / di;
    ma = max(ma, an);
    an = (d3 + d) / di;
    mi = min(mi, an - (double)0.000001);
  }
  int x = 0;
  x = (ma * (n + 1) - a[n] * 10) / 10;
  int t = 1;
  for (int i = ma + 1; i <= mi; i++) {
    int y = (i * (n + 1) - a[n] * 10) / 10;
    if (y != x) {
      cout << "not unique";
      return 0;
    }
  }
  int y = (mi * (n + 1) - a[n] * 10) / 10;
  if (y != x) {
    cout << "not unique";
    return 0;
  }
  cout << "unique" << endl;
  cout << a[n] + x;
  return 0;
}
