import syㄋ

line = sys.stdin.read()

line = line.splitlines()

N = int(line.pop(0))

entire_line = []

for i in range(N):

    entire_line.append(line[i])

a_line = []

def super_line_dddd_d():

    global a_line

    p =[]

    for i in entire_line:

        k = i.split()

        for j in k:

            p.append(int(j))

        a_line.append(p)

        p = []

super_line_dddd_d()

b_line = 0

c_line = 0

d_line = 0

e = 0

g = 0

a_line.sort()

for i in a_line:

    if g != 0:

        if i[1] - d_line >= 0:

            if i[1] - i[0] + b_line <= i[1] - c_line:

                e += i[1] - i[0]

            else:

                e += i[1] - d_line

        else:

            continue

    else:

        e += i[1] - i[0]

    b_line = i[1] - i[0]

    c_line = i[0]

    d_line = i[1]

    g += 1

print(e)

這個程式主要依靠兩個不相連的線段，最後減最前會>兩個線段相加的總長度的定理