//#pragma GCC optimize("Ofast","inline","-ffast-math")
//#pragma GCC target("avx,sse2,sse3,sse4,mmx")
//#pragma GCC optimize(2)
#include<bits/stdc++.h>
#define ll long long
#define inf 1e8
#define rg register
using namespace std;
const int N=1e5+10;
inline ll read(){
	register ll s=0,f=0;
	register char ch=getchar();
	while(!isdigit(ch)) f|=(ch=='-'),ch=getchar();
	while(isdigit(ch)) s=(s<<1)+(s<<3)+(ch^48),ch=getchar();
	return f?-s:s;
}
inline void write(ll x){
    if(x<0) putchar('-'),x=-x;
    if(x>9) write(x/10);
    putchar(x%10+'0');
}
int n,w,a[N],b[N];
ll maxn;
int main() {
//	freopen("in.txt","r",stdin);
//	freopen("out.txt","w",stdout);
	n=read(),w=read();
	for(int i=1;i<=n;i++)a[i]=read();
	for(int i=1;i<=n;i++)b[i]=read();
	int p=n-1,q=n-1;
	ll sa=a[n],sb=b[n],cnt=2;
	maxn=max(maxn,min(sa-cnt*w,sb-cnt*w));
	while(p&&q){
		if(p==0){
			sb+=b[q];
			q--;
			cnt++;
			maxn=max(maxn,min(sa-cnt*w,sb-cnt*w));
		}
		else if(q==0){
			sa+=a[p];
			p--;
			cnt++;
			maxn=max(maxn,min(sa-cnt*w,sb-cnt*w));
		}
		else if(sa>sb){
			sb+=b[q];
			q--;
			cnt++;
			maxn=max(maxn,min(sa-cnt*w,sb-cnt*w));
		}
		else {
			sa+=a[p];
			p--;
			cnt++;
			maxn=max(maxn,min(sa-cnt*w,sb-cnt*w));
		}
	}
	printf("%lld",maxn);
	return 0;
}