#include <iostream>
#include <vector>
#include <array>

using node = std::array<int, 26>;

int main() {
    int N, M;
    std::cin >> N >> M;
    std::vector<node> trie(1);
    while (N--) {
        std::string S;
        std::cin >> S;
        int curr = 0;
        for (char c : S) {
            if (trie[curr][c - 'A'] == 0) {
                trie[curr][c - 'A'] = trie.size();
                trie.emplace_back();
            }
            curr = trie[curr][c - 'A'];
        }
    }

    int curr = 0;
    std::string result;
    while (M--) {
        if (curr == -1) {
            result.push_back('G');
            continue;
        }
        if (trie[curr]['G' - 'A'] && trie[curr]['C' - 'A'] && trie[curr]['P' - 'A']) {
            std::cout << -1 << std::endl;
            return 0;
        }
        if (trie[curr]['G' - 'A'] && !trie[curr]['C' - 'A'] && !trie[curr]['P' - 'A']) {
            result.push_back('P');
            curr = -1;
            continue;
        }
        if (!trie[curr]['G' - 'A'] && trie[curr]['C' - 'A'] && !trie[curr]['P' - 'A']) {
            result.push_back('G');
            curr = -1;
            continue;
        }
        if (!trie[curr]['G' - 'A'] && !trie[curr]['C' - 'A'] && trie[curr]['P' - 'A']) {
            result.push_back('C');
            curr = -1;
            continue;
        }
        if (trie[curr]['G' - 'A'] && trie[curr]['C' - 'A']) {
            result.push_back('G');
            curr = trie[curr]['G' - 'A'];
            continue;
        }
        if (trie[curr]['G' - 'A'] && trie[curr]['P' - 'A']) {
            result.push_back('P');
            curr = trie[curr]['P' - 'A'];
            continue;
        }
        if (trie[curr]['P' - 'A'] && trie[curr]['C' - 'A']) {
            result.push_back('C');
            curr = trie[curr]['C' - 'A'];
            continue;
        }
        result.push_back('G');
    }
    if (curr != -1) std::cout << -1 << std::endl;
    else std::cout << result << std::endl;
}