#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
#define st first
#define nd second
const int maxn = 109;
const int maxAsumval = maxn*(maxn*maxn+maxn)/2+123;
const int zer = maxAsumval/2;
ll A[maxn*2];
ll L;
int dp[maxAsumval];
int dp2[maxAsumval];
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
int M;
cin >> M >> L;
for(int i = 0; i < 2*M+1 ; i++)
{
cin >> A[i];
}
//A[i] ma A[i-M] tak naprawde
for(int i = 0 ; i < maxAsumval ; i++)
{
dp[i] = -maxn*maxAsumval;
dp2[i] = -maxn*maxAsumval;
}
dp[0] = 0;
dp2[0] = 0;
for(int i = 0; i < M ; i++)
{
for(int l = 0 ; l < A[i] ; l++)
{
for(int j = maxAsumval-1 ; j >= 0 ; j--)
{
if((j-abs(i-M)) < 0)continue;
dp[j] = max(dp[j] , dp[j-abs(i-M)]+1);
}
}
}
for(int i = M; i < M*2+1 ; i++)
{
for(int l = 0 ; l < A[i] ; l++)
{
for(int j = maxAsumval-1 ; j >= 0 ; j--)
{
if((j-(i-M)) < 0)continue;
dp2[j] = max(dp2[j] , dp2[j-(i-M)]+1);
}
}
}
if(L == 0)
{
cout << dp[L] << '\n';
exit(0);
}
if(abs(L) >= maxAsumval)
{
cout << "impossible\n";
exit(0);
}
dp[0] = dp2[0];
int maxi = -maxAsumval;
for(int i = 0 ; i < maxAsumval ; i++)
{
if(i+L >= maxAsumval)continue;
maxi = max(maxi , dp[i] + dp2[i+L]);
}
if(maxi < 0)cout << "impossible\n";
else cout << maxi << '\n';
return 0;
}
