N,D = map(int,input().split())
A = [list(map(int,input().split())) for _ in range(N)]
A.insert(0,0)
dp = [[0 for _ in range(2)] for _ in range(N+1)]
dp[1][0] = A[1][0]
dp[1][1] = A[1][1]-D
for i in range(2,N+1):
    for j in range(2):
        dp[i][j] = dp[i-1][j]+A[i][j]
        dp[i][j] = max(dp[i][j],dp[i-1][1-j]+A[i][j]-D)
print(max(dp[N][0],dp[N][1]))