#include <iostream>
#include <math.h>
#include <string.h>
#include <vector>
#include <map>

long long nChoosek(int n, int k)
{
    if (k > n) return 0;
    if (k * 2 > n) k = n-k;
    if (k == 0) return 1;

    long long result = n;
    for( int i = 2; i <= k; ++i ) {
        result *= (n-i+1);
        result /= i;
    }
    return result;
}

void func5() {
    int n, m;
    std::cin >> n;
    std::cin >> m;
    char table[n][m];
    for (int i = 0; i < n + 1; i++) {
        std::string row;
        std::getline(std::cin, row);
        for (int j = 0; j < m; j++) {
            table[i - 1][j] = row[j];
        }
    }
    int l;
    std::cin >> l;
    int sum = n + m - 2;
    int caseCounter = 0;
    int i = 0, j = 0;
    bool flag = false;
    std::map<char, std::vector<std::pair<int, int>>> steps;
    std::vector<std::pair<int, int>> newStep;
    newStep.emplace_back(std::make_pair(0, 0));
    steps[table[0][0]] = newStep;
    std::string res = "";
    res += table[0][0];
    for (int t = 0; t < sum; t++) {
        std::map<char, std::vector<std::pair<int, int>>> newSteps;
        for (std::map<char, std::vector<std::pair<int, int>>>::iterator it = steps.begin(); it != steps.end(); it++){
            for (std::vector<std::pair<int, int>>::iterator it2 = it->second.begin(); it2 != it->second.end(); it2++) {
                if (it2->first + 1 < n) {
                    std::map<char, std::vector<std::pair<int, int>>>::iterator it3 = newSteps.find(table[it2->first + 1][it2->second]);
                    if (it3 != newSteps.end()) {
                        it3->second.emplace_back(std::make_pair(it2->first + 1, it2->second));
                    } else {
                        newStep.clear();
                        std::vector<std::pair<int, int>> newVec;
                        newStep.emplace_back(std::make_pair(it2->first + 1, it2->second));
                        newSteps[table[it2->first + 1][it2->second]] = newStep;
                    }
                }
                if (it2->second + 1 < m) {
                    std::map<char, std::vector<std::pair<int, int>>>::iterator it3 = newSteps.find(table[it2->first][it2->second + 1]);
                    if (it3 != newSteps.end()) {
                        it3->second.emplace_back(std::make_pair(it2->first, it2->second + 1));
                    } else {
                        newStep.clear();
                        std::vector<std::pair<int, int>> newVec;
                        newStep.emplace_back(std::make_pair(it2->first, it2->second + 1));
                        newSteps[table[it2->first][it2->second + 1]] = newStep;
                    }
                }
            }
        }
        for (std::map<char, std::vector<std::pair<int, int>>>::iterator it = newSteps.begin(); it != newSteps.end(); it++){
            int counter = 0;
            for (std::vector<std::pair<int, int>>::iterator it2 = it->second.begin(); it2 != it->second.end(); it2++) {
                counter += nChoosek(sum - it2->first - it2->second, n - 1 - it2->first);
            }
            if (l <= counter) {
                steps.clear();
                steps[it->first] = it->second;
                res += it->first;
                break;
            }
            l -= counter;
        }
    }
    std::cout << res;
}

int main()
{
    func5();
    return 0;
}