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

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

print(max(dp[-1]))