#include <stdio.h>

int main()
{
	int i, H, W;
	char S[3001][3001];
	scanf("%d %d", &H, &W);
	for (i = 0; i < H; i++) {
		scanf("%s", S[i]);
		S[i][W] = 'z' + 2;
	}
	for (i = 0; i < W; i++) S[H][i] = 'z' + 2;
	
	int j, k, q[2][3001][2], head, tail, cur, prev, flag[3001][3001] = {};
	char ans[6002], tmp;
	for (k = 1, cur = 1, prev = 0, ans[0] = S[0][0], q[0][0][0] = 0, q[0][0][1] = 0, tail = 1; k < H + W - 1; k++, cur ^= 1, prev ^= 1) {
		for (head = tail - 1, tail = 0, tmp = 'z' + 1; head >= 0; head--) {
			i = q[prev][head][0];
			j = q[prev][head][1];
			if (tmp > S[i+1][j]) {
				tmp = S[i+1][j];
				tail = 0;
			}
			if (tmp == S[i+1][j] && flag[i+1][j] == 0) {
				flag[i+1][j] = 1;
				q[cur][tail][0] = i + 1;
				q[cur][tail++][1] = j;
			}
			if (tmp > S[i][j+1]) {
				tmp = S[i][j+1];
				tail = 0;
			}
			if (tmp == S[i][j+1] && flag[i][j+1] == 0) {
				flag[i][j+1] = 1;
				q[cur][tail][0] = i;
				q[cur][tail++][1] = j + 1;
			}
		}
		ans[k] = tmp;
	}
	ans[H+W-1] = 0;
	printf("%s\n", ans);
	fflush(stdout);
	return 0;
}