#include <bits/stdc++.h>
using namespace std;
int n, a[400 + 2], ans = 1 << 30;
int main() {
  cin >> n;
  for (register int i = 1; i <= n; i++) cin >> a[i];
  for (int i = n; i >= 2; i--) {
    if (a[i] >= ans) break;
    for (int j = i; j <= n; j++) {
      register int num = a[i - 1] - 1, x = 1, t;
      for (register int k = i; k <= j; k++)
        if (num >= a[k]) {
          t = num / a[k];
          x += t, num -= t * a[k];
        }
      num = a[i - 1] - 1 - num + a[j];
      int res = num;
      if (res < ans) {
        for (register int k = 1; k <= n; k++)
          if (num >= a[k]) {
            t = num / a[k];
            x -= t, num -= t * a[k];
            if (x < 0) break;
          }
        if (x < 0) ans = res;
      }
    }
  }
  if (ans == 1 << 30) ans = -1;
  cout << ans << endl;
  return 0;
}
