N,D = map(int,input().split())
t,k = map(int,input().split())
#1日目
T = t
K = k -D
#i日目
for _ in range(N-1):
    t,k = map(int,input().split())
    tmp = T
    T = t + max(T,K-D)
    K = k + max(tmp-D,K)
print(max(T,K))