folia.cpp

#include <bits/stdc++.h>
using namespace std;

#define int long long
#define vi vector<int>
#define ii pair<int,int>
#define fi first
#define sc second
#define stoi stoll
#define popcnt __builtin_popcount
#define getbit(x, k) ((x >> k) & 1)
#define all(x) (x).begin(),(x).end()
#define FOR(i,j,k) for(int i=j; i<k; i++)
#define look(a) cerr <<#a<<": "<<a<<endl;
#define look2(a,b) cerr <<#a<<": "<<a<<" | "<<#b<<": "<<b<< endl;

void solve()
{
    int n;
    cin >> n;
    int a[n+1],b[n+1],c[n+1];
    FOR(i,0,n+1) cin >> a[i];
    if (a[0] > 1){
        cout << -1;
        return;
    }
    c[n] = a[n];
    for (int i = n-1; i >= 0; i--){
        c[i] = c[i+1] + a[i];
    }
    b[0] = 1 - a[0];
    FOR(i,1,n+1){
        if (a[i] > b[i-1]*2){
            cout << -1;
            return;
        }
        b[i] = min(c[i+1],b[i-1]*2 - a[i]);
        //look2(c[i+1],b[i-1]*2 - a[i])
        if (a[i] + b[i] < b[i-1]){
            cout << -1;
            return;
        }
    }
    //FOR(i,0,n+1) look2(a[i],b[i]);
    int s = a[n];
    FOR(i,0,n) s += a[i] + b[i];
    cout << s;
}

signed main()
{
    cin.tie(0)->sync_with_stdio(0);
    freopen("in", "r", stdin);
    freopen("out", "w", stdout);
    int T = 1;
    // cin >> T;
    while (T--) {
        solve();
        cout << '\n';
    }
    cerr << "Time elapsed: " << 1.0 * clock() / CLOCKS_PER_SEC << " s.\n";
}