#include <bits/stdc++.h>
using namespace std;
const int N = 110;
char a[N][N];
const double eps = 1e-8;
int n;
struct Node {
    int x, y;
    char c;
};
vector<Node> q;
bool check(Node a, Node b, Node c) {
    if (a.c != b.c || a.c != c.c || b.c != c.c) return false;
    double ax = (n - a.x + (a.y - 1) * 2);
    double bx = (n - b.x + (b.y - 1) * 2);
    double cx = (n - c.x + (c.y - 1) * 2);
    double ay = (n - a.x) * sqrt(3);
    double by = (n - b.x) * sqrt(3);
    double cy = (n - c.x) * sqrt(3);
    double c1 = (ax - bx) * (ax - bx) + (ay - by) * (ay - by);
    double c2 = (ax - cx) * (ax - cx) + (ay - cy) * (ay - cy);
    double c3 = (bx - cx) * (bx - cx) + (by - cy) * (by - cy);
    if (abs(c1 - c2) < eps && abs(c1 - c3) < eps && abs(c2 - c3) < eps) return true;
    return false;
}
int main() {
    cin >> n;
    string s;
    cin >> s;
    int idx = 0;
    for (int i = 1; i <= n; i++)
        for (int j = 1; j <= i; j++)
            a[i][j] = s[idx++];
    for (int i = 1; i <= n; i++)
        for (int j = 1; j <= i; j++)
            q.push_back({i, j, a[i][j]});
    vector<char> res;
    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++)
                if (check(q[i], q[j], q[k])) res.push_back(q[i].c);
    if (res.size() == 0)
        cout << "No Solution" << endl;
    else{
        sort(res.begin(), res.end());
        for (char x : res) cout << x;
    }
    return 0;
}