/*
 * 後ろから 2 つ目を見忘れる
 */
#include<cstddef>
#include<cstdio>
#include<cassert>
#include<iostream>
#include<vector>
template<class T> using numr=std::numeric_limits<T>;
using i64 = std::int_least64_t;
using usize = std::size_t;
auto cmn=[](auto&x,auto y){if(x>y){x=y;return true;}return false;};

template <class T>
T read_int(char end, T min, T max) {
    T x = 0;
    while (true) {
        char c = std::getchar();
        if (!std::isdigit(c)) {
            assert(c==end);
            break;
        }
        x = 10*x + c-'0';
        assert(x<=100'000'000);
    }
    assert(min <= x && x <= max);
    return x;
}

int main(){
    usize n = read_int<usize>(' ', 1u, 100'000u);
    i64 k = read_int<i64>('\n', 0ll, 100'000'000ll);

    i64 inf = numr<i64>::max();
    std::vector<i64> a(n), b(n), time(n, inf);
    for (usize i=0; i<n; i++) {
        a.at(i) = read_int<i64>(i==n-1?'\n':' ', 0ll, 100'000'000ll);
    }
    for (usize i=0; i<n; i++) {
        b.at(i) = read_int<i64>(i==n-1?'\n':' ', 0ll, 100'000'000ll);
    }
    assert(std::getchar()==EOF);
    time.at(0) = 0;


    for (usize i=1; i<n; i++) {
        time.at(i) = time.at(i-1)+a.at(i-1)+b.at(i);
        if (1<i) cmn(time.at(i), time.at(i-2)+a.at(i-2)+b.at(i)+k);
    }

    std::cout << (n==1? i64{0} : std::max(time.at(n-2), time.at(n-1))) << '\n';
}