#include <bits/stdc++.h>
using namespace std;
#define all(a) a.begin(),a.end()
#define owo ios_base::sync_with_stdio(0);cin.tie(0);
typedef long long int ll;
ll A[5000],B[5000];
unordered_map<ll,ll>val;
ll dp[2000001];
int main()
{
int m;
ll L;
cin>>m>>L;
for(int i=-m;i<=m;i++)cin>>A[i+m];
ll ans = A[m];
/*
//subtask 2 m^4 knapsack??
//consider left and right side separately
//let ai be number of art piece i chosen
//1)atleast one of ai , a_{-i} must be 0
//2)consider only positive
//ai aj i<j
//if aj >= i and ai <= Ai-j
//do ai+=j aj-=i
//j > i
//ai <= Ai-M and aj >= M cant be both true
//a prefix p has ai>Ai-M then p+1 can be anything
//ahhhhhhhhh iterate first i such that ai <= M
for(int i=1;i<=m;i++){
}
*/
ll shift = 500500;
if(L > shift || L < -shift){
cout<<"impossible"<<'\n';
return 0;
}
for(int i=0;i<=2000000;i++)dp[i] = -1;
dp[shift] = 0;
for(ll i=-m;i<=m;i++){
if(i==0)continue;
ll z = 1;
vector<ll>item;
while(A[i+m]>=z){
item.push_back(z);
A[i+m]-=z;
z*=2;
}
if(A[i+m])item.push_back(A[i+m]);
if(i < 0){
for(ll x:item){
for(ll j=-shift-x*i;j<=shift;j++){
if(dp[j+shift] != -1)dp[j + x*i+shift] = max(dp[j + x*i+shift],dp[j+shift] + x);
}
}
}else{
for(ll x:item){
for(ll j=shift-x*i;j>=-shift;j--){
if(dp[j+shift] != -1)dp[j + x*i+shift] = max(dp[j + x*i+shift],dp[j+shift] + x);
}
}
}
}
if(dp[L+shift] == -1){
cout<<"impossible"<<'\n';
return 0;
}
ans+=dp[L+shift];
cout<<ans<<'\n';
}
