from dataclasses import dataclass, field
from typing import Optional


@dataclass
class MinimumManager:
    value: Optional[str] = None
    idxes: set[int] = field(default_factory=set)


def main():
    H, W = map(int, input().split())
    letters = [list(input()) for _ in range(H)]

    step_letters = []
    for idx_sum in range(H+W-1):
        step_letters.append([])
        for idx_height in range(idx_sum+1):
            try:
                step_letters[-1].append(letters[idx_height]
                                        [idx_sum-idx_height])
            except IndexError:
                pass

    strings = ""
    current_idx = set((0,))
    for step_idx, step_letter in enumerate(step_letters):
        minimum = MinimumManager()
        for idx in current_idx:
            if minimum.value is None:
                minimum.value = step_letter[idx]
                minimum.idxes = set((idx, ))
                continue

            if minimum.value == step_letter[idx]:
                minimum.idxes.add(idx)
            elif minimum.value > step_letter[idx]:
                minimum.value = step_letter[idx]
                minimum.idxes = set((idx,))
        strings += minimum.value

        current_idx = set()
        for idx in minimum.idxes:
            if step_idx < max(W, H) - 1:
                current_idx.add(idx)
                current_idx.add(min(idx+1, min(H, W)-1))
            else:
                current_idx.add(min(idx, len(step_letter)-2))
                current_idx.add(max(idx-1, 0))

    print(strings)


if __name__ == "__main__":
    main()