//Zory-2019
#include<bits/stdc++.h>
using namespace std;
namespace mine
{
    typedef long long ll;
    #define pr pair<int,int>
    #define FR first
    #define SE second
    #define MP make_pair
    #define PB push_back
    #define vc vector
    #define all(x) (x).begin(),(x).end()
    #define sz(x) ((int)(x).size())
    #define bin(x) (1ll<<(x))
    #define GG(x) if(x) {puts("error");exit(666);}
    #define fo(i,l,r) for(int i=(l),I=(r);i<=I;i++)
    #define fd(i,r,l) for(int i=(r),I=(l);i>=I;i--)
    ll qread()
    {
        ll ans=0,f=1;char c=getchar();
        while(c<'0' or c>'9') {if(c=='-')f=-1;c=getchar();}
        while('0'<=c and c<='9') ans=ans*10+c-'0',c=getchar();
        return ans*f;
    }
    void write(ll num)
    {
        if(num<0) putchar('-'),num=-num;
        if(num>=10) write(num/10);
        putchar('0'+num%10);
    }
    void write1(ll num){write(num);putchar(' ');}
    void write2(ll num){write(num);putchar('\n');}
    template<typename T> void chmax(T &x,const T y) {x=(x>y?x:y);}
    template<typename T> void chmin(T &x,const T y) {x=(x<y?x:y);}
    ll gcd(ll x,ll y){return y?gcd(y,x%y):x;}

    const int INF=0x3f3f3f3f;
    const int MOD=998244353;
    int mm(const int x){return x>=MOD?x-MOD:x;}
    template<typename T> void add(T &x,const int &y){x=(x+y>=MOD?x+y-MOD:x+y);}
    ll qpower(ll x,ll e,int mod=MOD){ll ans=1;GG(e<0)while(e){if(e&1)ans=ans*x%mod;x=x*x%mod;e>>=1;}return ans;}
    ll invm(ll x){return qpower(x,MOD-2);}
    const int N=1e6+10;

    int p[N],s[N];ll f[2][N];
	void main()
	{
        int n=qread(),C=qread();fo(i,1,n) p[i]=qread();fo(i,1,n) s[i]=qread();
        fo(i,1,n)
        {
            int op=i&1;f[op][0]=f[op^1][0]+p[i],f[op][i]=f[op^1][i-1]+s[i];
            fo(cnt,1,i-1) f[op][cnt]=min(f[op^1][cnt]+1ll*cnt*C+p[i],f[op^1][cnt-1]+s[i]);
        }
        ll ans=bin(62);fo(cnt,0,n) chmin(ans,f[n&1][cnt]);write(ans);
	}
};//(ans+MOD)%MOD
signed main()
{
    // freopen("a.in","r",stdin);
    srand(time(0));
    mine::main();
}