def main():
    import sys
    input = sys.stdin.read().split()
    idx = 0
    N = int(input[idx])
    idx +=1
    M = int(input[idx])
    idx +=1

    grid = []
    for _ in range(N):
        grid.append(input[idx])
        idx +=1

    # Compute all accessible buttons
    accessible = []
    for i in range(N):
        for j in range(M):
            if grid[i][j] == '.':
                accessible.append( (i,j) )

    K = len(accessible)
    if K % 2 != 0:
        print("NO")
        return

    # Compute black and white counts
    black = 0
    white = 0
    for (i,j) in accessible:
        if (i + j) % 2 == 0:
            black +=1
        else:
            white +=1

    if black == white:
        print("YES")
    else:
        print("NO")

if __name__ == "__main__":
    main()