N, D = map(int, input().split())
TK = [[int(_) for _ in input().split()] for _ in range(N)]
dp = [[0]*2 for _ in range(N+1)]
dp[1][0] = TK[0][0]
dp[1][1] = TK[0][1] - D
for i in range(2, N+1):
    dp[i][0] = max(dp[i-1][0]+TK[i-1][0], dp[i-1][1]+TK[i-1][0]-D)
    dp[i][1] = max(dp[i-1][0]+TK[i-1][1]-D, dp[i-1][1]+TK[i-1][1])

print(max(dp[N][0], dp[N][1]))