a1paper.cpp

#include <iostream>
#include <vector>
#include <cmath>

using namespace std;

typedef long long ll;
typedef long double ld;

int main() {
    ll n;
    cin >> n;
    n--;

    vector<ll> v(n);
    for(auto& i : v) {
        cin >> i;
    }

    ld totalcost = 0;
    ll needed = 1;
    bool enough = false;

    ld longedge  = pow(2.0, -3/4.0);
    ld shortedge = pow(2.0, -5/4.0);

    for(ll i = 0; i < v.size(); i++) {
        // Calculate cost here
        totalcost += needed * longedge;
        swap(shortedge, longedge);
        shortedge /= 2;

        // Calculate paper needed now
        needed *= 2;
        needed -= v[i];

        // Check if we have enough paper
        if(needed <= 0) {
            enough = true;
            break;
        }
    }

    cout << fixed;
    cout.precision(9);

    if(enough) {
        cout << totalcost << endl;
    }
    else {
        cout << "impossible" << endl;
    }
}