#include #define rep(i,n) for (int i = 0; i < (n); ++i) #define rrep(i,n) for (int i = (n)-1; i >= 0; i--) #define rep2(i,s,n) for (int i = (s); i < (n); ++i) #define all(a) a.begin(),a.end() #define rall(a) a.rbegin(),a.rend() #define pb push_back #define eb emplace_back #define vi vector #define vvi vector> #define vl vector #define vvl vector> #define vd vector #define vs vector #define vc vector #define vb vector #define vp vector

using namespace std; using ll = long long; using P = pair; using LP = pair; template istream& operator>>(istream &is,pair &p) { return is >> p.first >> p.second; } template ostream& operator<<(ostream &os,const pair &p) { return os<<'{'< istream& operator>>(istream &is,vector &v) { for(T &t:v){is>>t;} return is; } template ostream& operator<<(ostream &os,const vector &v) { os<<'[';rep(i,v.size())os< bool chmin(T& a,T b) {if(a > b){a = b; return true;} return false;} template bool chmax(T& a,T b) {if(a < b){a = b; return true;} return false;} const int inf = 1001001001; const ll linf = 1001001001001001001; int main() { ios::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr); int n,k; cin >> n >> k; vi a(n),b(n); cin >> a >> b; vl ans(n); ans[0] = 0; rep2(i,1,n) { if(i == 1) ans[i] = ans[i-1]+a[i-1]+b[i]; else ans[i] = min(ans[i-1]+a[i-1]+b[i],ans[i-2]+a[i-2]+b[i]+k); } cout << ans[n-1] << endl; }