N,D = map(int,input().split())
T =[0]*N
K =[0]*N
for i in range(N):
    T[i],K[i]=map(int,input().split())
DPT = [0]*(N+1)
DPK = [0]*(N+1)
DPK[0] = -10**15
for i in range(N):
    DPT[i+1] = max(DPT[i]+T[i],DPK[i]+T[i]-D) 
    DPK[i+1] = max(DPK[i]+K[i],DPT[i]+K[i]-D)


print(max(DPT[N],DPK[N]))