h, w = map(int, input().split())
S = [list(input()) for _ in range(h)]
Directions = [(1, 0), (0, 1)]

ans = S[0][0]
C = set([0])
for _ in range(h + w - 2):
    minT = ''
    NC = set()
    for c in C:
        ci, cj = divmod(c, w)
        for di, dj in Directions:
            ni = ci + di
            nj = cj + dj
            if ni < h and nj < w:
                if minT == '':
                    minT = S[ni][nj]
                    NC.add(ni * w + nj)
                elif S[ni][nj] == minT:
                    NC.add(ni * w + nj)
                elif S[ni][nj] < minT:
                    minT = S[ni][nj]
                    NC = set([ni * w + nj])
    ans += minT
    C = NC
print(ans)