N,D = [int(i) for i in input().split()]
A = 0
Lo = "T"
for i in range(N):
    T,K = [int(i) for i in input().split()]
    if Lo == "T" and K-T <= D:
        A += T
    elif Lo == "T" and K-T >D:
        A += K
        A -= D
        Lo = "K"
    elif Lo == "K" and T-K <= D:
        A += K
    else:
        A += T
        A -= D
        Lo = "T"
print(A)