N,D = list(map(int,input().split()))
T = [0] * N
K = [0] * N
for i in range(N):
    T[i],K[i] = list(map(int,input().split()))
dp = [[0] * 2 for _ in range(N)]

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