original article URL : http://blog.163.com/eden_284914869/blog/static/252246078201691042223315/

here is the code:

#include <cstdio>
#include <cstring>
#include <iostream>
#include <cmath>
#include <string>
#include <cstdlib>
#define longlongtype
using namespace std;
typedef long long LL;
#define ref(i,x,y)for(int i=x;i<=y;i++)
#define def(i,x,y)for(int i=x;i>=y;i--)
#ifdef longlongtype
	typedef long long LL;
	#define T LL
	const T oo=1e16;
#endif
#ifdef inttype
	#define T int
	const T oo=2e9;
#endif
T max(T a,T b){return a>b?a:b;};
T min(T a,T b){return a<b?a:b;};
T gcd(T a,T b){return b?gcd(b,a%b):a;};
T read(){char c=getchar();T d=0,f=1;for(;c<'0'||c>'9';c=getchar())if(c=='-')f=-1;for(;c>='0'&&c<='9';d=d*10+c-48,c=getchar());return d*f;}
T n,c,ans,a[10001],b[10001],f[10001];
int main()
{
	n=read(),c=read();
	ref(i,1,n)a[i]=read();
	ref(i,1,n)b[i]=read();
	ref(i,1,n)f[i]=oo;
	ref(i,1,n)def(j,i-1,0)
	{
		f[j+1]=min(f[j+1],f[j]+b[i]);
		f[j]=f[j]+a[i]+j*c;
	}
	ans=oo;
	ref(i,0,n)ans=min(ans,f[i]);
	printf("%I64d\n",ans);
}