N,D=map(int,input().split())
T=[]
K=[]
for i in range(N):
    t,k=map(int,input().split())
    T.append(t)
    K.append(k)

dp=[[0,0] for i in range(N+1)]
dp[1][0]=T[0]
dp[1][1]=K[0]-D

for i in range(1,N):
    dp[i+1][0]=max(dp[i][0]+T[i],dp[i][1]+T[i]-D)
    dp[i+1][1]=max(dp[i][0]+K[i]-D,dp[i][1]+K[i])

print(max(dp[N]))