_input = input().split('\n')
h,w = _input[0].split(' ')
grid = list(map(lambda x: x.split(''),_input[1:]))
index = [0,0]
result = ''
for _ in range(h+w-1):
    if grid[index[0]+1][index[1]]>grid[index[0]][index[1]+1]:
        result += grid[index[0]][index[1]+1]
        index[1] += 1
    else:
        result += grid[index[0]+1][index[1]]
        index[0] += 1