H,W = map(int,input().split())
S = [list(input()) for _ in range(H)]
INFTY = 10**9
D = {chr(i):i-96 for i in range(97,123)}
D[INFTY] = INFTY
Pa = {}
dp = [[INFTY for _ in range(W)] for _ in range(H)]
dp[0][0] = D[S[0][0]]
Pa[(0,0)] = -1
for i in range(H):
    for j in range(W):
        if i+1<H and j+1<W and D[S[i][j+1]]<=D[S[i+1][j]]:
            if dp[i][j]+D[S[i][j+1]]<dp[i][j+1]:
                dp[i][j+1] = dp[i][j]+D[S[i][j+1]]
                Pa[(i,j+1)] = (i,j)
        elif i+1<H and j+1<W and D[S[i][j+1]]>=D[S[i+1][j]]:
            if dp[i][j]+D[S[i+1][j]]<dp[i+1][j]:
                dp[i+1][j] = dp[i][j]+D[S[i+1][j]]
                Pa[(i+1,j)] = (i,j)
        elif i==H-1 and j+1<W:
            if dp[i][j]+D[S[i][j+1]]<dp[i][j+1]:
                dp[i][j+1] = dp[i][j]+D[S[i][j+1]]
                Pa[(i,j+1)] = (i,j)
        elif i+1<H and j==W-1:
            if dp[i][j]+D[S[i+1][j]]<dp[i+1][j]:
                dp[i+1][j] = dp[i][j]+D[S[i+1][j]]
                Pa[(i+1,j)] = (i,j)
T = []
cur = (H-1,W-1)
while cur!=-1:
    T.append(S[cur[0]][cur[1]])
    cur = Pa[cur]
print("".join(T[::-1]))