N,D = map(int,input().split())
income = [tuple(map(int,input().split())) for _ in range(N)]
dp0,dp1 = 0,-10**10
for T,K in income:
    today0 = max(dp0 + T,dp1 - D + T)
    today1 = max(dp1 + K,dp0 - D + K)
    dp0,dp1 = today0,today1
ans = max(dp0,dp1)
print(ans)