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

dp = [[0]*2 for _ in range(N+1)]
dp[0][1] = -D
for i in range(N):
    for j in range(2):
        dp[i+1][j] = max(dp[i][j], dp[i][j^True] - D) + TK[i][j] 
print(max(dp[N]))