#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <cmath>
#include <queue>
#include <stack>
#include <map>
#include <set>
#include <vector>
#include <iomanip>
#include <sstream>
#include <bitset>
#include <unordered_map>
// #include <bits/stdc++.h>
#define ALL(x) (x).begin(), (x).end()
#define sz(a) int(a.size())
#define rep(i,x,n) for(int i=x;i<n;i++)
#define repd(i,x,n) for(int i=x;i<=n;i++)
#define pii pair<int,int>
#define pll pair<long long ,long long>
#define gbtb ios::sync_with_stdio(false),cin.tie(0),cout.tie(0)
#define MS0(X) memset((X), 0, sizeof((X)))
#define MSC0(X) memset((X), '\0', sizeof((X)))
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define eps 1e-6
#define chu(x)  if(DEBUG_Switch) cout<<"["<<#x<<" "<<(x)<<"]"<<endl
#define du3(a,b,c) scanf("%d %d %d",&(a),&(b),&(c))
#define du2(a,b) scanf("%d %d",&(a),&(b))
#define du1(a) scanf("%d",&(a));
using namespace std;
typedef long long ll;
ll gcd(ll a, ll b) {return b ? gcd(b, a % b) : a;}
ll lcm(ll a, ll b) {return a / gcd(a, b) * b;}
ll powmod(ll a, ll b, ll MOD) { if (a == 0ll) {return 0ll;} a %= MOD; ll ans = 1; while (b) {if (b & 1) {ans = ans * a % MOD;} a = a * a % MOD; b >>= 1;} return ans;}
ll poww(ll a, ll b) { if (a == 0ll) {return 0ll;} ll ans = 1; while (b) {if (b & 1) {ans = ans * a ;} a = a * a ; b >>= 1;} return ans;}
void Pv(const vector<int> &V) {int Len = sz(V); for (int i = 0; i < Len; ++i) {printf("%d", V[i] ); if (i != Len - 1) {printf(" ");} else {printf("\n");}}}
void Pvl(const vector<ll> &V) {int Len = sz(V); for (int i = 0; i < Len; ++i) {printf("%lld", V[i] ); if (i != Len - 1) {printf(" ");} else {printf("\n");}}}
inline long long readll() {long long tmp = 0, fh = 1; char c = getchar(); while (c < '0' || c > '9') {if (c == '-') fh = -1; c = getchar();} while (c >= '0' && c <= '9') tmp = tmp * 10 + c - 48, c = getchar(); return tmp * fh;}
inline int readint() {int tmp = 0, fh = 1; char c = getchar(); while (c < '0' || c > '9') {if (c == '-') fh = -1; c = getchar();} while (c >= '0' && c <= '9') tmp = tmp * 10 + c - 48, c = getchar(); return tmp * fh;}
void pvarr_int(int *arr, int n, int strat = 1) {if (strat == 0) {n--;} repd(i, strat, n) {printf("%d%c", arr[i], i == n ? '\n' : ' ');}}
void pvarr_LL(ll *arr, int n, int strat = 1) {if (strat == 0) {n--;} repd(i, strat, n) {printf("%lld%c", arr[i], i == n ? '\n' : ' ');}}
const int maxn = 100010;
const ll inf = 1e18;
/*** TEMPLATE CODE * * STARTS HERE ***/
#define DEBUG_Switch 0
int n;
ll a[maxn];
ll b[maxn];
ll tree[maxn];
int lowbit(int x)
{
    return (-x)& x;
}
void add(int pos, ll val)
{
    while (pos < maxn)
    {
        tree[pos] += val;
        pos += lowbit(pos);
    }
}
ll ask(int pos)
{
    ll res = 0;
    while (pos > 0)
    {
        res += tree[pos];
        pos -= lowbit(pos);
    }
    return res;
}

int main()
{
#if DEBUG_Switch
    freopen("C:\\code\\input.txt", "r", stdin);
#endif
    //freopen("C:\\code\\output.txt","r",stdin);
    n = readint();
    repd(i, 0, n)
    {
        a[i] = readint();
    }
    if (n == 0)
    {
        if (a[0] == 1)
        {
            printf("1\n");
        } else
        {
            printf("-1\n");
        }
    } else
    {
        if (a[0] != 0)
        {
            printf("-1\n");
        } else
        {
            b[0] = 1;
            int isok = 1;
            repd(i, 1, n)
            {
                ll x = b[i - 1] << 1;
                x = min(x, inf);
                if (a[i] > x)
                {
                    isok = 0;
                    break;
                } else
                {
                    b[i] = x - a[i];
                }
            }
            if (isok)
            {
                // printf("Yes\n");
                b[0]--;
                repd(i, 1, n)
                {
                    b[i] += a[i];
                    b[i]--;
                    add(i, b[i] - b[i - 1]);
                }
                int id = 1;
                ll ans = n + 1;
                a[n]--;
                repd(i, 1, n)
                {
                    while (a[i] > 0)
                    {
                        ll num = ask(id);
                        while (num == 0 && id <= n)
                        {
                            id++;
                            num = ask(id);
                        }
                        num = min(num, a[i]);
                        ans += 1ll * (i - id + 1) * num;
                        add(id, -num);
                        add(i + 1, num);
                        a[i] -= num;
                    }
                }
                printf("%lld\n", ans );
            } else
            {
                printf("-1\n");
            }
        }
    }
    return 0;
}




Measured memory_bytes: 6004