CCC 2021 S1 Crazy Fencing

    1#include <bits/stdc++.h>
    2 
    3using namespace std;
    4 
    5int main() {
    6    int n;
    7    cin >> n;
    8    vector<int> w (n + 1);
    9    for(int i = 0; i < w.size(); i++){
    10        cin >> w[i];
    11    }
    12    vector<int> h(n);
    13    for(int i = 0; i < h.size(); i++){
    14        cin >> h[i];
    15    }
    16    int index = 0;
    17    double ans = 0;
    18    for(int i = 0; i < w.size() - 1; i++){
    19        ans += h[index] * (w[i] + w[i + 1]) / 2.0;
    20        index++;
    21    }
    22    cout << fixed << setprecision(6) << ans << endl;
    23    return 0;
    24}
    25