#include <bits/stdc++.h>
const int N = 110;
char a[N][N];
/*
4
abccddadca
*/
using namespace std;

struct Node {
    int x, y;
    char c;
};
vector<Node> q;

int main() {
#ifndef ONLINE_JUDGE
    freopen("ZhiMuSanJiaoXing.in", "r", stdin);
#endif
    int n;
    cin >> n;
    string s;
    cin >> s;
    // 先把字符串存入char[][]
    int idx = 0;
    for (int i = 0; i < n; i++)      // n行
        for (int j = 0; j <= i; j++) // i列
            a[i][j] = s[idx++];

    // 把所有坐标记录下来
    for (int i = 0; i < n; i++) {
        for (int j = 0; j <= i; j++)
            q.push_back({i, j, a[i][j]});
    }
    vector<char> res;
    // 在q数组中，选择任意三个，计算两两间距离是不是相等
    for (int i = 0; i < q.size(); i++)
        for (int j = i + 1; j < q.size(); j++)
            for (int k = j + 1; k < q.size(); k++) {
                Node x = q[i], y = q[j], z = q[k];
                if (check()) res.push_back(x.c);
            }

    if (res.size() == 0)
        cout << "No Solution" << endl;
    else {
        sort(res.begin(), res.end());
        for (char x : res) cout << x << endl;
    }
    return 0;
}