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

dp = [TK[0][0], TK[0][1]-D]

for i in range(1, N):
    a = max(dp[0]+TK[i][0], dp[1]+TK[i][0] - D)
    b = max(dp[1]+TK[i][1], dp[0]+TK[i][1] - D)
    dp[0], dp[1] = a, b

print(max(dp))