#include <string>
#include <vector>
#include <iostream>
using namespace std;

int main() {
	int H, W;
	cin >> H >> W;
	vector<string> S(H);
	for (int i = 0; i < H; i++) {
		cin >> S[i];
	}
	vector<vector<int> > f(H, vector<int>(W, 0));
	f[0][0] = 1;
	string answer(1, S[0][0]);
	for (int i = 0; i < H + W - 2; i++) {
		char minch = '~';
		for (int x = 0; x < H; x++) {
			int y = i - x;
			if (!(0 <= y && y < W)) {
				continue;
			}
			if (f[x][y] == 1) {
				if (x != H - 1) {
					minch = min(minch, S[x + 1][y]);
				}
				if (y != W - 1) {
					minch = min(minch, S[x][y + 1]);
				}
			}
		}
		for (int x = 0; x < H; x++) {
			int y = i - x;
			if (!(0 <= y && y < W)) {
				continue;
			}
			if (f[x][y] == 1) {
				if (x != H - 1 && S[x + 1][y] == minch) {
					f[x + 1][y] = 1;
				}
				if (y != W - 1 && S[x][y + 1] == minch) {
					f[x][y + 1] = 1;
				}
			}
		}
		answer += minch;
	}
	cout << answer << endl;
	return 0;
}