pipioj 1241: 01背包（中途相遇法）

#define bug(x) cout<<#x<<" is "<<x<<endl
#define IO std::ios::sync_with_stdio(0)
#include <bits/stdc++.h>
using namespace  std;
typedef long long ll;
#define mk make_pair
#define pb push_back
const int inf=2147483647;
const int N=1e5+10;
ll w,ans;
ll a[N],v[N];
int cnt,n,e;
void dfs(int h,ll x){
    if(h>e){
        a[++cnt]=x;
        return;
    }
    dfs(h+1,x);
    dfs(h+1,x+v[h]);
}
void dfs1(int h,ll x){
    if(h>e){
        int k=upper_bound(a+1,a+1+cnt,w-x)-a;
        ans+=k-1;
        return;
    }
    dfs1(h+1,x);
    dfs1(h+1,x+v[h]);
}
int main(){
    IO;
    while(cin>>n>>w){
        for(int i=1;i<=n;i++){
            cin>>v[i];
        }
        ans=cnt=0;
        e=n/2;
        dfs(1,0);
        sort(a+1,a+1+cnt);
        e=n;
        dfs1(n/2+1,0);
        cout<<ans<<endl;
    }
}
/*
30 100
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

*/