#include <bits/stdc++.h>
using namespace std;
#define int ll
#define ll long long
#define pii pair<ll, ll>
#define f first
#define s second
#define SZ(x) (int)((x).size())
#define ALL(x) (x).begin(), (x).end()
#define pb push_back
#define FOR(i,a,b) for (int i = a; i<b; ++i)
#define RREP(i,n) for (int i = n-1; i>=0;--i)
#define REP(i,n) FOR(i,0,n)
#define REP1(i,n) FOR(i,1,n+1)
#define MX(a,b) a = max(a,b)
#define MN(a,b) a = min(a,b)
#ifdef BALBIT
#define bug(...) cerr<<"#"<<__LINE__<<": "<<#__VA_ARGS__<<"- ", _do(__VA_ARGS__)
template<typename T> void _do( T && x) {cerr<<x<<endl;}
template<typename T,typename ...S> void _do( T && x, S && ...y) {cerr<<x<<", "; _do(y...);}
#else
#define bug(...)
#define endl '\n'
#endif // BALBIT
ll amt[606];
ll taken[606];
const ll inf = 0x3f3f3f3f3f3f3f3f;
const int rng = 300 * 300 ;
ll bst[rng*2+4];
vector<ll> bd(ll x) {
// maybe i can min x ith rng or smth
MN(x, rng*2);
vector<ll> re;
ll scale = 1;
while(x) {
if (x & 1) {
re.pb(scale); x-=1;
}else{
re.pb(scale); re.pb(scale); x-=2;
}
x/=2; scale *= 2;
}
return re;
}
void ASS(bool x) {
if (!x) while(1);
}
signed main(){
ios::sync_with_stdio(0), cin.tie(0);
bug(1,2);
// vector<int> hi = bd(23);
// for (int x : hi) cout<<x<<' ';
// return 0;
REP(i, rng*2+4) {
bst[i] = -inf;
}
int m; ll L;
cin>>m>>L;
ll tot = 0;
ll things= 0;
FOR(i,-m,m+1) {
cin>>amt[i+m];
tot += i * (amt[i+m]);
things += amt[i+m];
}
bug(tot, L);
if (tot < L) {
// flip everything
FOR(i,0,m+1) {
swap(amt[i+m], amt[-i+m]);
}
tot = -tot; L = -L;
}
FOR(i,-m,m+1) {
taken[i+m] = amt[i+m];
}
bug(tot,L);
for (int i = m; i>=1; --i) {
if (tot > L + rng) {
int need = (tot - (L+rng)-1) / i + 1; // ceiling
need = min(need, taken[i+m]);
taken[i+m] -= need; tot -= i * need; things -= need;
}
if (tot <= L+rng && tot >= L-rng) {
// MX(bst[tot-L+rng], things);
bug(tot, things);
while (taken[i+m] && (tot-i >= L-rng)) {
--taken[i+m]; tot -= i; --things;
bug(tot, things);
// MX(bst[tot-L+rng], things);
}
}
}
bug(tot, L, things);
if (tot > L) {
// sad
cout<<"impossible"<<endl; return 0;
}
MX(bst[tot-L+rng], things);
// FOR(j,-2*m,2*m+1) {
// bug(j, bst[j+rng]);
// }
FOR(i,1,m+1) {
if (amt[i+m] - taken[i+m]) {
// try adding stuff back
for (ll d : bd(amt[i+m] - taken[i+m])) {
if (i < 0) {
ASS(0);
}else{
RREP(j, rng*2+1 - d*i) {
if (j + d*i < 0) ASS(0);
if (j + d*i > rng*2) ASS(0);
MX(bst[j+d*i], bst[j]+d);
}
}
}
}
#ifdef BALBIT
// bug(i, taken[i+m], amt[i+m]);
// FOR(j,-2*m,2*m+1) {
// bug(j, bst[j+rng]);
// }
#endif
}
FOR(i,-m,m+1) {
if (i == 0) continue;
if (taken[i+m]) {
// try removing stuff
for (ll d : bd(taken[i+m])) {
if (i < 0) {
RREP(j, rng*2+1+d*i) {
if (j - d*i < 0) ASS(0);
if (j - d*i > rng*2) ASS(0);
MX(bst[j-d*i], bst[j]-d);
}
}else{
FOR(j, d*i, rng*2+1) {
if (j - d*i < 0) ASS(0);
if (j - d*i > rng*2) ASS(0);
MX(bst[j-d*i], bst[j]-d);
}
}
}
}
#ifdef BALBIT
// bug(i, taken[i+m], amt[i+m]);
// FOR(j,-m,m+1) {
// bug(j, bst[j+rng]);
// }
#endif
}
if (bst[rng] <= 0) cout<<"impossible"<<endl;
else cout<<bst[rng]<<endl;
}
