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