1
n, x = map(int, input().split())
2
 
3
cur = 1
4
ans = 0
5
 
6
for i in range(n):
7
    l, r = map(int, input().split())
8
    while cur + x <= l:
9
        cur += x
10
    ans += r - cur + 1
11
    cur = r + 1
12
    
13
print(ans)