import sys
readline = sys.stdin.readline

def alp_to_num(cha):
    num = ord(cha) - ord("a")
    return num


H, W = map(int, readline().split())
S = []
for i in range(H):
    L = list(readline().rstrip())
    L = list(map(alp_to_num, L))
    S.append(L)
    
x, y = 0, 0
pre = [[] for i in range(26)]
pre[S[0][0]].append(0)
dx, dy = [1, 0], [0, 1]
ans = []
for i in range(H + W - 1):
    dp = [[] for i in range(26)]
    SS = set()
    for i, lst in enumerate(pre):
        if lst:
            ans.append(chr(i + ord("a")))
            for z in lst:
                nx, ny = divmod(z, W)
                for k in range(2):
                    x = nx + dx[k]
                    y = ny + dy[k]
                    if x == H or y == W:
                        continue
                    if (x, y) in SS:
                        continue
                    SS.add((x, y))
                    dp[S[x][y]].append(x * W + y)
            break
    dp, pre = pre, dp
print(*ans, sep="")