#include <bits/stdc++.h>
int n, cnt[3]; char S[10010];
inline int solve (int x)
{
    int y = (x + 1) % 3, z = (y + 1) % 3, cx = cnt[x], cy = cnt[y], cz = cnt[z];
    if (!cx and (!cy or !cz)) return 1 << 30;
    return ((cy + cz) & 1) ? (1 << 30) : std::max(cy, cz);
}
signed main ()
{
    while (~scanf("%d", &n))
    {
        scanf("%s", S); cnt[0] = cnt[1] = cnt[2] = 0;
        for (int i = 0; i < n; i++) cnt[S[i] - 'A']++;
        int w[3] = {solve(0), solve(1), solve(2)};
        int ans = std::min({w[0], w[1], w[2]}); std::vector<char> Ans;
        for (int i = 0; i < 3; i++) if (ans == w[i]) Ans.push_back('A' + i);
        putchar((int)Ans.size() > 1 ? 'N' : Ans[0]); putchar('\n');
    }
    return 0;
}