#include <iostream>
using namespace std;
const int N = 300, B = N * (N + 1);
const long long INF = 1e18;
namespace dq {
pair<long long, int> d[B * 2 + 1];
int u, v, l, r;
void clear() { u = v = l = r = 0; }
int size() { return v - u; }
void push(long long x) {
while (r - l > 0 && d[r - 1].first <= x) r--;
d[r++] = {x, v++};
}
void pop() {
if (d[l].second == u++) l++;
}
long long query() { return d[l].first; }
} // namespace dq
void process(long long *f, int b, int i, int c, int t) {
if (i > 0) {
for (int md = 0; md < i; md++) {
dq::clear();
for (int j = md; j <= b * 2; j += i) {
dq::push(f[j] == -INF ? -INF : f[j] - j / i * t);
if (dq::size() > c + 1) dq::pop();
f[j] = dq::query() == -INF ? -INF : dq::query() + j / i * t;
}
}
} else if (i < 0) {
i = -i;
for (int md = 0; md < i; md++) {
dq::clear();
for (int j = (b * 2 - md) / i * i + md; j >= 0; j -= i) {
dq::push(f[j] == -INF ? -INF : f[j] + j / i * t);
if (dq::size() > c + 1) dq::pop();
f[j] = dq::query() == -INF ? -INF : dq::query() - j / i * t;
}
}
} else if (t > 0) {
for (int j = 0; j <= b * 2; j++)
if (f[j] != -INF) f[j] += c * t;
}
}
int main() {
ios::sync_with_stdio(0), cin.tie(0);
int n;
long long m;
cin >> n >> m;
static long long k[N * 2 + 1], u[N * 2 + 1];
long long m_ = 0;
for (int i = -n; i <= +n; i++) {
cin >> k[n + i];
u[n + i] = k[n + i], m_ += i * k[n + i];
}
if (m_ < m) {
for (int i = n; i > 0 && m_ < m; i--) {
long long t = min(u[n - i], (m - m_ + i - 1) / i);
m_ += t * i, u[n - i] -= t;
}
if (m_ < m) {
cout << "impossible\n";
return 0;
}
} else if (m_ > m) {
for (int i = n; i > 0 && m_ > m; i--) {
long long t = min(u[n + i], (m_ - m + i - 1) / i);
m_ -= t * i, u[n + i] -= t;
}
if (m_ > m) {
cout << "impossible\n";
return 0;
}
}
int b = n * (n + 1);
static long long f[B * 2 + 1];
for (int i = -b; i <= +b; i++) f[b + i] = -INF;
f[b + 0] = 0;
for (int i = -n; i <= +n; i++) {
process(f, b, -i, min(u[n + i], (long long)n + 1), -1);
process(f, b, +i, min(k[n + i] - u[n + i], (long long)n + 1), +1);
}
if (f[b + m - m_] == -INF) {
cout << "impossible\n";
return 0;
}
long long ans = f[b + m - m_];
for (int i = -n; i <= +n; i++) ans += u[n + i];
cout << ans << '\n';
return 0;
}
