#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const ll N = 610,M = 4e5,INF = 1e18;
template<typename T>void tomin(T& x,T y){x = min(x,y);}
template<typename T>void tomax(T& x,T y){x = max(x,y);}
//
int n,m; ll L,a[N],b[N],sum;

ll dp[M];

void add_item(int v,int w){
    if(v > 0){
        for(int i=m-v;i>=-m;i--)if(dp[i+m] != -INF)tomax(dp[i+v+m],dp[i+m] + w);
    }else{
        v *= -1;
        for(int i=-m+v;i<=m;i++)if(dp[i+m] != -INF)tomax(dp[i-v+m],dp[i+m] + w);
    }
}
void add_item(int v,int w,ll num){
    num = min(2*m,(int)num); if(!num)return;
    for(int cnt=1;cnt<=num;num -= cnt,cnt*=2){
        add_item(v*cnt,w*cnt);
    }
    if(num)add_item(v*num,w*num);
}

int main(){
    cin>>n>>L; m = 2*n*n;
    for(int i=-n;i<=n;i++)cin>>a[i+n]; sum += a[n];
    for(int i=-n;i<=-1;i++)L += (-i) * a[i+n],sum += a[i+n];
    if(L < 0){
        cout<<"impossible\n";
        return 0;
    }
    for(int i=1;i<=n;i++){
        ll num = min(a[n+i],L/i);
        L -= i*num,sum += num,a[n+i] -= num,b[n+i] = num;
    }
    for(int i=n;i>=1;i--){
        ll num = min(a[n-i],L/i);
        L -= i*num,sum -= num,a[n-i] -= num,b[n-i] = num;
    }
    if(L > m){
        cout<<"impossible\n";
        return 0;
    }
    //
    for(int i=-m;i<=m;i++)dp[i+m] = -INF;
    dp[m] = 0;
    for(int i=1;i<=n;i++){
        add_item(i,1,a[n+i]);
        add_item(-i,-1,b[n+i]);
        add_item(i,-1,a[n-i]);
        add_item(-i,1,b[n-i]);
    }
    if(dp[L+m] == -INF)cout<<"impossible\n";
    else cout<<dp[L+m] + sum<<"\n";

    return 0;
}