tink

class Graph(object):

    def __init__(self, graph_dict=None):
        """ initializes a graph object
            If no dictionary or None is given,
            an empty dictionary will be used
        """
        if graph_dict == None:
            graph_dict = {}
        self.__graph_dict = graph_dict

    def vertices(self):
        """ returns the vertices of a graph """
        return list(self.__graph_dict.keys())

    def edges(self):
        """ returns the edges of a graph """
        return self.__generate_edges()

    def add_vertex(self, vertex):
        """ If the vertex "vertex" is not in
            self.__graph_dict, a key "vertex" with an empty
            list as a value is added to the dictionary.
            Otherwise nothing has to be done.
        """
        if vertex not in self.__graph_dict:
            self.__graph_dict[vertex] = []

    def add_edge(self, edge):
        """ assumes that edge is of type set, tuple or list;
            between two vertices can be multiple edges!
        """
        edge = set(edge)
        (vertex1, vertex2) = tuple(edge)
        if vertex1 in self.__graph_dict:
            self.__graph_dict[vertex1].append(vertex2)
        else:
            self.__graph_dict[vertex1] = [vertex2]

    def __generate_edges(self):
        """ A static method generating the edges of the
            graph "graph". Edges are represented as sets
            with one (a loop back to the vertex) or two
            vertices
        """
        edges = []
        for vertex in self.__graph_dict:
            for neighbour in self.__graph_dict[vertex]:
                if {neighbour, vertex} not in edges:
                    edges.append({vertex, neighbour})
        return edges

    def __str__(self):
        res = "vertices: "
        for k in self.__graph_dict:
            res += str(k) + " "
        res += "\nedges: "
        for edge in self.__generate_edges():
            res += str(edge) + " "
        return res

    def find_all_paths(self, start_vertex, end_vertex, path=[]):
        """ find all paths from start_vertex to
            end_vertex in graph """
        graph = self.__graph_dict
        path = path + [start_vertex]
        if start_vertex == end_vertex:
            return [path]
        if start_vertex not in graph:
            return []
        paths = []
        for vertex in graph[start_vertex]:
            if vertex not in path:
                extended_paths = self.find_all_paths(vertex,
                                                     end_vertex,
                                                     path)
                for p in extended_paths:
                    paths.append(p)
        return paths

n, m = map(int, input().split())

d = {}
q = {}
data = []
string = ""
for i in range(n):
    s = str(input())
    string += s
    if i > 0:
        for j in range(m - 1):
            d[(i-1) * m + j] = [i * m + j, (i - 1) * m + j + 1]

        d[(i - 1) * m + (m - 1)] = [i * m + (m - 1)]

for i in range(m - 1):
    d[m * (n - 1) + i] = [m * (n - 1) + i + 1]

graph = Graph(d)
path = graph.find_all_paths(0, n * m - 1)

for i in path:
    s = ''
    for j in i:
        s += string[j]

    bisect.insort(data, s)
l = int(input())
print(data[l - 1])