#include <bits/stdc++.h>
using namespace std;
using namespace chrono;
#if __has_include(<atcoder/all>)
#include <atcoder/all>
using namespace atcoder;
#endif

int main() {
    int64_t n;
    cin >> n;
    vector<int64_t> as(n);
    for (auto &&a : as) {
        cin >> a;
    }

    for (int64_t i = 0; i < n - 1; i++) {
        vector<int64_t> bs;
        for (int64_t j = 0; j + 1 < as.size(); j++) {
            bs.push_back(as[j] + as[j + 1]);
        }
        as = bs;
    }

    cout << as[0] << endl;

    return 0;
}