N, D = map(int, input().split())

pos = 0
sum = 0
for i in range(N):
    t, k = map(int, input().split())
    
    if pos == 0:
        if k - D > t:
            pos = 1
            sum += k - D
        else:
            sum += t 
            
    if pos == 1:
        if t - D > k:
            pos = 0
            sum += t - D
        else:
            sum += k

print(sum)