N, D = map(int, input().split())
A = [list(map(int, input().split())) for _ in range(N)]

dp = [[0]*2 for _ in range(N+2)]
dp[-1][1] = -10**9
for i in range(N):
    dp[i][0] = max(dp[i-1][0], dp[i-1][1]-D)+A[i][0]
    dp[i][1] = max(dp[i-1][1], dp[i-1][0]-D)+A[i][1]

print(max(dp[N-1]))