h,w = map(int,input().split())
s = [input() for i in range(h)]

# BFSっぽいのをする. (ただし少し特殊)

mada = [(0,0)]
t = [0] * h*w
t[0] = 1
while mada:
	k = 26
	for x, y in mada:
		if x < h-1:
			k = min(k, ord(s[x+1][y]) - ord("a"))
		if y < w-1:
			k = min(k, ord(s[x][y+1]) - ord("a"))
	fr = []
	for x, y in mada:
		if x < h-1:
			if ord(s[x+1][y]) - ord("a") == k:
				if t[(x+1)*w + y] == 0:
					fr.append((x+1, y))
					t[(x+1)*w + y] = 1
		if y < w-1:
			if ord(s[x][y+1]) - ord("a") == k:
				if t[x*w + y+1] == 0:
					fr.append((x, y+1))
					t[x*w + y+1] = 1
	mada = fr

ans = []
x = h-1; y = w-1
while x > 0 or y > 0:
	if x > 0 and t[(x-1)*w + y] == 1:
		ans.append(s[x][y])
		x -= 1
	elif y > 0 and t[x*w + y-1] == 1:
		ans.append(s[x][y])
		y -= 1

ans.append(s[0][0])

print("".join(ans[::-1]))