WATERMOV

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

using ld = long double;

const int maxN = 1e6 + 5;

struct point {
    ld x, y;
};

bool CCW(point a, point b, point c) { // Ngược chiều kim đồng hồ
    return a.x * (c.y-b.y) + b.x * (a.y-c.y) + c.x * (b.y-a.y) < 0;
}

ld Stugiac(point a, point b, ld subc) {
    return (b.x - a.x) * (b.y + a.y - 2 * subc);
}

int n;
point pot[maxN];

int main() {
#ifdef LOCAL
    freopen("in1.txt", "r", stdin);
#else
    freopen("WATERMOV.inp", "r", stdin);
    freopen("WATERMOV.out", "w", stdout);
#endif
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);

    cin >> n;
    pot[1] = {0, 0};
    n++;
    ld sum = 0;
    for (int i = 2; i <= n; i++) {
        ld a; cin >> a;
        sum += a;
        pot[i] = {(i - 1), sum};
    }

    // Convex-Hull
    point p1 = pot[1], p2 = pot[n];
    vector<int> down;
    down.push_back(1);
    for (int i = 2; i <= n; i++) {
        if (i == n || CCW(p1, pot[i], p2)) {
            while (down.size() >= 2 && !CCW(pot[down[down.size() - 2]], pot[down[down.size() - 1]], pot[i]))
                down.pop_back();
            down.push_back(i);
        }
    }

    ld sall = 0, sdet = 0;
    for (int i = 2; i <= n; i++)
        sall += Stugiac(pot[i - 1], pot[i], pot[1].y);
    for (int i = 1; i < down.size(); i++)
        sdet += Stugiac(pot[down[i - 1]], pot[down[i]], pot[down[0]].y);

    sall -= sdet;
    cout << fixed << setprecision(1) << sall / 2.0 << '\n';

    return 0;
}