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