N,D = map(int,input().split())
t,k = map(int,input().split())
dp=[[t,k-D],[0,0]]
for _ in range(N-1):
    t,k= map(int,input().split())
    dp[1][0] = t + max(dp[0][0],dp[0][1]-D)
    dp[1][1] = k + max(dp[0][0]-D,dp[0][1])
print(max(dp[1]))