N, D = list(map(int, input().split()))
dp = [[0 for i in range(2)] for j in range(N+1)]
dp[0][1] = -float('inf')

for i in range(1, N+1):
    t, k = list(map(int, input().split()))
    dp[i][0] = max(dp[i-1][0], dp[i-1][1]-D) + t
    dp[i][1] = max(dp[i-1][0]-D, dp[i-1][1]) + k

print(max(dp[N]))