from collections import defaultdict
H, W = map(int, input().split())
S = [input() for _ in range(H)]

dp = {S[0][0]: {(0, 0)}}
for i in range(H+W-2):
    ndp = defaultdict(set)
    mins = 'z'*(i+1)
    for s in dp:
        for h, w in dp[s]:
            if h < H-1:
                nt = s+S[h+1][w]
                ndp[nt].add((h+1, w))
                mins = min(mins, nt)
            if w < W-1:
                nt = s+S[h][w+1]
                ndp[nt].add((h, w+1))
                mins = min(mins, nt)
    dp = {mins: {t for t in ndp[mins]}}

for k in dp:
    print(k)
    exit()