#include using ll = long long; using uint = unsigned int; using ull = unsigned long long; using ld = long double; template using max_heap = std::priority_queue; template using min_heap = std::priority_queue, std::greater>; constexpr int popcount(const ull v) { return v ? __builtin_popcountll(v) : 0; } constexpr int log2p1(const ull v) { return v ? 64 - __builtin_clzll(v) : 0; } constexpr int lsbp1(const ull v) { return __builtin_ffsll(v); } constexpr int clog(const ull v) { return v ? log2p1(v - 1) : 0; } constexpr ull ceil2(const ull v) { return 1ULL << clog(v); } constexpr ull floor2(const ull v) { return v ? (1ULL << (log2p1(v) - 1)) : 0ULL; } constexpr bool btest(const ull mask, const int ind) { return (mask >> ind) & 1ULL; } template void bset(T& mask, const int ind) { mask |= ((T)1 << ind); } template void breset(T& mask, const int ind) { mask &= ~((T)1 << ind); } template void bflip(T& mask, const int ind) { mask ^= ((T)1 << ind); } template void bset(T& mask, const int ind, const bool b) { (b ? bset(mask, ind) : breset(mask, ind)); } template bool chmin(T& a, const T& b) { return (a > b ? a = b, true : false); } template bool chmax(T& a, const T& b) { return (a < b ? a = b, true : false); } template constexpr T inf_v = std::numeric_limits::max() / 4; template constexpr Real pi_v = Real{3.141592653589793238462643383279502884}; constexpr ull TEN(const int n) { return n == 0 ? 1ULL : TEN(n - 1) * 10ULL; } template struct fix : F { fix(F&& f) : F{std::forward(f)} {} template auto operator()(Args&&... args) const { return F::operator()(*this, std::forward(args)...); } }; class printer { public: printer(std::ostream& os_ = std::cout) : os{os_} {} template int operator()(const T& v) { return os << v, 0; } template int operator()(const std::vector& vs) { for (int i = 0; i < (int)vs.size(); i++) { os << (i ? " " : ""), this->operator()(vs[i]); } return 0; } template int operator()(const std::vector>& vss) { for (int i = 0; i < (int)vss.size(); i++) { os << (0 <= i or i + 1 < (int)vss.size() ? "\n" : ""), this->operator()(vss[i]); } return 0; } template int operator()(const T& v, const Args&... args) { return this->operator()(v), os << ' ', this->operator()(args...), 0; } template int ln(const Args&... args) { return this->operator()(args...), os << '\n', 0; } template int el(const Args&... args) { return this->operator()(args...), os << std::endl, 0; } template int fmt(const std::string& s, const Args&... args) { return rec(s, 0, args...); } private: int rec(const std::string& s, int index) { return os << s.substr(index, s.size()), 0; } template int rec(const std::string& s, int index, const T& v, const Args&... args) { return index == s.size() ? 0 : s[index] == '%' ? (this->operator()(v), rec(s, index + 1, args...)) : (os << s[index], rec(s, index + 1, v, args...)); } std::ostream& os; }; printer out; template std::vector generated(const int n, F f) { std::vector ans(n); return std::generate(ans.begin(), ans.end(), f), ans; } std::vector ioted(const int n, const int offset = 0) { std::vector ans(n); return std::iota(ans.begin(), ans.end(), offset), ans; } template Vs reversed(const Vs& vs) { auto ans = vs; return std::reverse(ans.begin(), ans.end()), ans; } template> std::vector sorted(const std::vector& vs, F comp = F{}) { auto ans = vs; return std::sort(ans.begin(), ans.end(), comp), ans; } template std::vector sorted_iota(const int n, F comp = F{}, const int offset = 0) { return sorted(ioted(n, offset), comp); } class scanner { public: scanner(std::istream& is_ = std::cin) : is{is_} { is.tie(nullptr), std::ios::sync_with_stdio(false); } template T val() { static T v; return is >> v, v; } template T val(const T offset) { return val() - offset; } template std::vector vec(const int n) { return generated(n, [&] { return val(); }); } template std::vector vec(const int n, const T offset) { return generated(n, [&] { return val(offset); }); } template std::vector> vvec(const int n0, const int n1) { return generated>(n0, [&] { return vec(n1); }); } template std::vector> vvec(const int n0, const int n1, const T offset) { return generated>(n0, [&] { return vec(n1, offset); }); } template auto tup() { return std::tuple...>{val()...}; } template auto tup(const Args&... offsets) { return std::tuple...>{val(offsets)...}; } private: std::istream& is; }; scanner in; # define SHOW(...) static_cast(0) template auto make_v(int const (&szs)[n], const T x = T{}) { if constexpr (i == n) { return x; } else { return std::vector(szs[i], make_v(szs, x)); } } int main() { const auto [N, K] = in.tup(); const auto as = in.vec(N); const auto bs = in.vec(N); std::vector ts{inf_v, as[0]}; for (int i = 1; i < N; i++) { ts.push_back(std::min(ts.back(), ts[ts.size() - 2] + K) + as[i] + bs[i]); }; ll ans = 0; for (int i = 0; i < N; i++) { chmax(ans, ts[i + 1] - as[i]); } out.ln(ans); return 0; }