#include <bits/extc++.h>
using namespace std;
using namespace __gnu_pbds;
#define int long long
#define ll long long
#define f first
#define s second
#define all(x) x.begin(), x.end()
#define debug(x) [&](decltype(x) _x) {cerr << #x << " = " << _x << endl; return _x;}(x)
using pii = pair<int, int>;
const ll inf = 1ll << 60;
// const int inf = 1 << 30;
template <typename T, typename U>
ostream& operator<<(ostream& out, pair<T, U> p) {
return out << "(" << p.f << ", " << p.s << ")";
}
template <typename T, typename = decltype(begin(declval<T>()))>
typename enable_if<!is_same<T, string>::value, ostream&>::type operator<<(ostream& out, T x) {
string dlm = "";
out << "{";
for(auto i : x) {
out << dlm << i;
dlm = ", ";
}
return out << "}";
}
const int K = 2;
const int K2 = 2;
const int K3 = 0;
signed main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
int m, l; cin >> m >> l;
int* a = new int[2 * m + 1];
a += m;
int tot = 0;
for(int i = -m; i <= m; i++) {
cin >> a[i];
tot += a[i];
l -= a[i] * i;
}
l *= -1;
if(l < 0) {
// debug("HERE");
l *= -1;
reverse(a - m, a + m + 1);
}
// debug(l);
int j = m;
int ans = 0;
while(l > m * m * K) {
while(a[j] <= m * K3) {
j--;
if(j <= 0) {
cout << "impossible" << endl;
return 0;
}
}
if(l - (a[j] - m * K3) * j > m * m * K) {
l -= (a[j] - m * K3) * j;
ans += (a[j] - m * K3);
a[j] = 0;
} else {
int v = (l - m * m * K) / j;
if(a[j] - v > m * K3) v++;
l -= v * j;
a[j] -= v;
ans += v;
}
}
vector<int> th;
for(int i = -m; i <= m; i++) {
if(i == 0) continue;
for(int j = 0; j < min(a[i], m); j++) {
th.push_back(i);
}
}
random_shuffle(all(th));
// debug(th);
int* dp = new int[m * m * K * K2 * 2 + 1];
dp += m * m * K * K2;
fill(dp - m * m * K * K2, dp + m * m * K * K2 + 1, inf);
dp[0] = 0;
for(auto i : th) {
if(i > 0) {
for(int j = m * m * K * K2; j >= -m * m * K * K2; j--) {
if(j - i >= -m * m * K * K2) {
dp[j] = min(dp[j], dp[j - i] + 1);
}
}
} else {
for(int j = -m * m * K * K2; j <= m * m * K * K2; j++) {
if(j - i <= m * m * K * K2) {
dp[j] = min(dp[j], dp[j - i] + 1);
}
}
}
}
if(dp[l] >= inf) {
cout << "impossible" << endl;
return 0;
}
cout << tot - (ans + dp[l]) << endl;
}
