def func(d, r, sharps):
    find = set()
    for y, x in sharps:
        if (y, x) in find:
            continue
        ny = y + d
        nx = x + r
        if (ny, nx) in sharps:
            find.add((y, x))
            find.add((ny, nx))
        else:
            return False

    return len(find) == len(sharps)


def main():
    h, w = map(int, input().split())

    sharps = []
    for y in range(h):
        line = input()
        for x in range(len(line)):
            if line[x] == '#':
                sharps.append((y, x))

    for d in range(50):
        for r in range(-50, 50):
            if d == 0 and r == 0:
                continue
            if func(d, r, sharps):
                print("YES")
                return
    print("NO")
    
if __name__ == '__main__':
    main()