# https://ja.wikipedia.org/wiki/%E3%83%9F%E3%83%A9%E3%83%BC%E2%80%93%E3%83%A9%E3%83%93%E3%83%B3%E7%B4%A0%E6%95%B0%E5%88%A4%E5%AE%9A%E6%B3%95
  class Integer
    def prime?
      n = self.abs()
      return true if n == 2
      return false if n == 1 || n & 1 == 0
      d = n-1
      d >>= 1 while d & 1 == 0
      10.times do
        a = rand(n-2) + 1
        t = d
        y = ModMath.pow(a,t,n)
        while t != n-1 && y != 1 && y != n-1
          y = (y * y) % n
          t <<= 1
        end
        return false if y != n-1 && t & 1 == 0
      end
      return true
    end
  end
  
  module ModMath
    def ModMath.pow(base, power, mod)
      result = 1
      while power > 0
        result = (result * base) % mod if power & 1 == 1
        base = (base * base) % mod
        power >>= 1;
      end
      result
    end
  end

N=gets.to_i
A=gets.split
p A.permutation.map(&:join).map(&:to_i).sort{|a,b|b<=>a}.uniq.find(&:prime?)||-1