def covered_length(intervals):
    ordered = sorted(intervals)
    if not ordered:
        return 0
    start, end = ordered[0]
    total = 0
    for next_start, next_end in ordered[1:]:
        if next_start <= end:
            end = max(end, next_end)
        else:
            total += end - start
            start, end = next_start, next_end
    return total + end - start