#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
using namespace std;
typedef long long int ll;
typedef pair<int,int> P;
#define yn {puts("YES");}else{puts("NO");}
#define MAX_N 200005

int main() {
    ll n, k;
    cin >> n >> k;
    ll a[n], b[n];
    rep(i,n)cin >> a[i];
    rep(i,n)cin >> b[i];

    ll dp[n][2];
    rep(i,n)rep(j,2)dp[i][j] = 0;

    rep(i,n){
        if(i == 0){
            dp[i][1] = a[i];
        }else if(i == 1){
            dp[i][0] = dp[i-1][1] + b[i];
            dp[i][1] = dp[i][0] + a[i];
        }else{
            dp[i][0] = min(dp[i-1][1], dp[i-2][1]+k) + b[i];
            dp[i][1] = dp[i][0] + a[i];
        }
    }

    cout << dp[n-1][0] << endl;
    return 0;
}