#include <bits/stdc++.h>
using std::sort;
using std::cin;
using std::cout;
using std::endl;
using std::string;

int n, flag, axis;
string inStr, ans;
struct point {
    int x, y;
    char v; // value
};
std::vector<point> pt;
struct point inf;

bool comp(const point &a, const point &b) {
    if (a.v == b.v) {
        return (a.x == b.x) ? (a.y < b.y) : (a.x < b.x);
    } else {
        return a.v < b.v;
    }
}
int main() {
    cin >> n >> inStr;
    flag = 0, axis = 0;
    for (register int count = 0; count < n; ++count) {
        for (register int countB = 0; countB < count + 1; ++countB) {
            point newPt;
            newPt.v = inStr[flag];
            newPt.y = count;
            newPt.x = axis + (countB << 1);
            ++flag;
            pt.push_back(newPt);
            // used for debug
            // cout<<newPt.v<<' '<<newPt.x<<' '<<newPt.y<<endl;
            // end debug
        }
        --axis;
    }
    inf.x = (1 << 8), inf.y = (1 << 8), inf.v = '~';
    pt.push_back(inf);
    sort(pt.begin(), pt.end(), comp);
    for (register int count = 0; count <= flag; ++count) {
        if (pt[count].v == '~') {
            break;
        }
        // used for debug
        // cout<<'*'<<pt[count].v<<' '<<pt[count-1].v<<' '<<pt[count-2].v<<endl;
        // end debug
        if (pt[count].v != pt[count + 1].v) {
            if (pt[count - 2].v == pt[count].v) {
                int dist[3];
                dist[0] = (pt[count - 2].x - pt[count - 1].x) * (pt[count - 2].x - pt[count - 1].x)
                          + 3 * (pt[count - 2].y - pt[count - 1].y) * (pt[count - 2].y - pt[count - 1].y);
                dist[1] = (pt[count - 2].x - pt[count].x) * (pt[count - 2].x - pt[count].x)
                          + 3 * (pt[count - 2].y - pt[count].y) * (pt[count - 2].y - pt[count].y);
                dist[2] = (pt[count - 1].x - pt[count].x) * (pt[count - 1].x - pt[count].x)
                          + 3 * (pt[count - 1].y - pt[count].y) * (pt[count - 1].y - pt[count].y);
                // used for debug
                // cout<<'*'<<pt[count].v<<' '<<dist[0]<<' '<<dist[1]<<' '<<dist[2]<<endl;
                // end debug
                if (dist[1] == dist[2] && dist[0] == dist[1]) {
                    ans = ans + pt[count].v;
                }
            }
        }
    }
    if (ans != "") {
        cout << ans << endl;
    } else {
        puts("No Solution");
    }
    return 0;
}