#include <bits/stdc++.h>
using namespace std;
const int maxn = 300005;
long long n, a[maxn], d, sum;
int Pow(int a, int t) {
  if (t == 0) return 1;
  if (t == 1) return a;
  int P = Pow(a, t / 2);
  return P * P * Pow(a, t % 2);
}
int main() {
  cin >> n;
  for (int i = 1; i <= n; i++) {
    cin >> a[i];
    d += (a[i] % 2) * Pow(-1, i);
    sum += a[i];
  }
  cout << (sum - abs(d)) / 2 << endl;
  return 0;
}
