from math import inf

N,D=map(int,input().split())
X=[0]*(N+1)
Y=[0]*(N+1)
Y[0]=-inf

for i in range(N):
    T,K=map(int,input().split())
    X[i+1]=max(X[i],Y[i]-D)+T
    Y[i+1]=max(X[i]-D,Y[i])+K

print(max(X[-1],Y[-1]))