from math import gcd
import sys

int1 = lambda x: int(x) - 1

# input = lambda: sys.stdin.buffer.readline()
input = lambda: sys.stdin.readline().rstrip()
ii = lambda: int(input())
i1 = lambda: int1(input())
mi = lambda: map(int, input().split())
mi1 = lambda: map(int1, input().split())
li = lambda: list(mi())
li1 = lambda: list(mi1())
lli = lambda n: [li() for _ in range(n)]

INF = float("inf")
mod = int(1e9 + 7)
# mod = 998244353

h, w = mi()
s = [input() for i in range(h)]
ans = [s[0][0]]
e = chr(ord("z") + 1)
d = [list() for i in range(h + w - 1)]
for y in range(h):
    for x in range(w):
        d[y + x].append(y * w + x)
f = [[False] * w for i in range(h)]
f[0][0] = True
for i in range(h + w - 2):
    mn = e
    for tmp in d[i]:
        y, x = tmp // w, tmp % w
        if not f[y][x]:
            continue
        if y + 1 < h and mn > s[y + 1][x]:
            mn = s[y + 1][x]
        if x + 1 < w and mn > s[y][x + 1]:
            mn = s[y][x + 1]
    ans.append(mn)
    for tmp in d[i]:
        y, x = tmp // w, tmp % w
        if not f[y][x]:
            continue
        if y + 1 < h and s[y + 1][x] == mn:
            f[y + 1][x] = True
        if x + 1 < w and s[y][x + 1] == mn:
            f[y][x + 1] = True
print("".join(ans))