def min_rho_difference(lines):
    """Being lines a list of lines (rho, theta) sorted according to rho,
       from low to high, returns the minimum difference between two
       consecutive lines. The size of the list must be at least two."""
    assert len(lines) >= 2
    min_diff = lines[1][0] - lines[0][0]
    for i in range(2, len(lines)):
        diff = lines[i][0] - lines[i - 1][0]
        if diff < min_diff:
            min_diff = diff
    return min_diff
