N,D=map(int,input().split())
T0,K0=0,-D
for _ in range(N):
    t,k=map(int,input().split())
    T1=max(T0+t,K0+t-D)
    K1=max(T0+k-D,K0+k)
    T0,K0=T1,K1   
print(max(T0,K0))