#include <bits/stdc++.h>
const int N = 1e5+10;
using i64 = long long;
int main()
{
    int n;
    std::cin >> n;
    std::vector<i64> a(n+10);
    for (int i = 1; i <= n; i ++) {
        std::cin >> a[i];
    }
    i64 res = 0;
    for (int i = 1, j = n; i <= j; i ++, j --) {
        int t = std::min(a[i], a[j]);
        a[i] -= t;
        if (i != j) a[j] -= t;
    }

    for (int i = 1; i <= n; i ++) {
        i64 t = a[i];
        res += a[i];
        a[i+1] -= std::min(t, a[i+1]);
    }
    std::cout << res << "\n";
    return 0;
}