#include <bits/stdc++.h>
using namespace std;
int main()
{
int64_t m, l;
cin >> m >> l;
vector<int64_t> a(2 * m + 1);
for (int64_t &x : a)
cin >> x;
if (l < 0)
{
l = -l;
for (int64_t i = 0; i < m; i++)
swap(a[m - i - 1], a[m + i + 1]);
}
int64_t p = 0, total_added = 0;
vector<int64_t> b(a.begin(), a.end());
for (int64_t i = -m; i <= 0; i++)
p += a[i + m] * i, b[i + m] = 0, total_added += a[i + m];
for (int64_t i = 1; i <= m; i++)
{
for (int64_t j = 62; j >= 0; j--)
if (1LL << j <= b[i + m] && p + (1LL << j) * i <= l)
{
p += (1LL << j) * i;
b[i + m] -= 1LL << j;
total_added += 1LL << j;
}
}
if (p < l - m)
{
p = 0;
total_added = 0;
b = vector<int64_t>(a.begin(), a.end());
for (int64_t i = 0; i <= m; i++)
p += a[i + m] * i, b[i + m] = 0, total_added += a[i + m];
for (int64_t i = -1; i >= -m; i--)
{
for (int64_t j = 62; j >= 0; j--)
if (1LL << j <= b[i + m] && p + (1LL << j) * i >= l)
{
p += (1LL << j) * i;
b[i + m] -= 1LL << j;
total_added += 1LL << j;
}
}
if (p < l - m)
{
cout << "impossible\n";
return 0;
}
}
// l resides at m * m.
vector<int64_t> dp1(2 * m * m + 1, 0), dp2(2 * m * m + 1, 0);
dp1[m * m - (l - p)] = total_added;
for (int64_t i = -m; i <= m; i++)
{
vector<int64_t> max_add(dp1.size(), min<int64_t>(b[i + m], m)),
max_remove = vector<int64_t>(dp1.size(), min<int64_t>(a[i + m] - b[i + m], m));
bool f = 1;
for (int64_t k = 0; f; k++)
{
f = 0;
for (int64_t j = 0; j < dp1.size(); j++)
{
if (!dp1[j] || !(0 <= j + (1LL << k) * i && j + (1LL << k) * i < dp1.size() && 1LL << k < max_add[j]))
continue;
f = 1;
dp2[j + (1LL << k) * i] = max<int64_t>(dp2[j + (1LL << k) * i], dp1[j] + (1LL << k));
max_add[j] -= 1LL << k;
}
swap(dp1, dp2);
for (size_t i = 0; i < dp1.size(); i++)
dp1[i] = max(dp1[i], dp2[i]), dp2[i] = max(dp1[i], dp2[i]);
}
for (int64_t j = 0; j < dp1.size(); j++)
if (dp1[j] && 0 <= j + max_add[j] * i && j + max_add[j] * i < dp1.size())
dp2[j + max_add[j] * i] = max<int64_t>(dp2[j + max_add[j] * i], dp1[j] + max_add[j]);
swap(dp1, dp2);
for (size_t i = 0; i < dp1.size(); i++)
dp1[i] = max(dp1[i], dp2[i]), dp2[i] = max(dp1[i], dp2[i]);
f = 1;
for (int64_t k = 0; f; k++)
{
f = 0;
for (int64_t j = 0; j < dp1.size(); j++)
{
if (!dp1[j] || !(0 <= j - (1LL << k) * i && j - (1LL << k) * i < dp1.size() && 1LL << k < max_remove[j]))
continue;
f = 1;
dp2[j - (1LL << k) * i] = max<int64_t>(dp2[j - (1LL << k) * i], dp1[j] - (1LL << k));
max_remove[j] -= 1LL << k;
}
swap(dp1, dp2);
for (size_t i = 0; i < dp1.size(); i++)
dp1[i] = max(dp1[i], dp2[i]), dp2[i] = max(dp1[i], dp2[i]);
}
for (int64_t j = 0; j < dp1.size(); j++)
if (dp1[j] && 0 <= j - max_remove[j] * i && j - max_remove[j] * i < dp1.size())
dp2[j - max_remove[j] * i] = max<int64_t>(dp2[j - max_remove[j] * i], dp1[j] - max_remove[j]);
swap(dp1, dp2);
for (size_t i = 0; i < dp1.size(); i++)
dp1[i] = max(dp1[i], dp2[i]), dp2[i] = max(dp1[i], dp2[i]);
}
if (!dp1[m * m])
cout << "impossible\n";
else
cout << dp1[m * m] << '\n';
}
