#include <bits/stdc++.h>

using namespace std;

#define fst(t) std::get<0>(t)
#define snd(t) std::get<1>(t)
#define thd(t) std::get<2>(t)
#define unless(p) if(!(p))
#define until(p) while(!(p))

using ll = long long;
using P = std::tuple<int,int>;

const int dx[8] = {-1, 1, 0, 0, -1, -1, 1, 1}, dy[8] = {0, 0, -1, 1, -1, 1, -1, 1};

int A[1000];

bool can(int n){
    int pr = n % 10;
    n /= 10;

    while(n > 0){
        int nx = n % 10;
        if(nx + 1 != pr){
            return false;
        }
        pr = nx;
        n /= 10;
    }

    return true;
}

int main(){
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);

    int N;
    std::cin >> N;

    for(int i=0;i<N;++i){
        std::cin >> A[i];
    }

    int res = -1;
    for(int i=0;i<N;++i){
        for(int j=i+1;j<N;++j){
            if(can(A[i] * A[j])){
                res = max(res, A[i] * A[j]);
            }
        }
    }

    std::cout << res << std::endl;
}

