#include <bits/stdc++.h>
using namespace std;
template <class t>
t mag(t a) {
  return a > 0 ? a : -a;
}
template <class T, class U>
T cdiv(T num, U den) {
  return (num / den) + ((num % den) > 0);
}
std::ifstream term("/dev/tty");
std::string tempstring;
using ll = int_fast64_t;
using ii = pair<int, int>;
using vi = vector<int>;
using vl = vector<ll>;
using vii = vector<ii>;
const int sij = 400010;
const ll mod = 1000000007ll;
int n;
int esc(int tcase) {
  cin >> n;
  ll tot[2] = {0, 0};
  int cr = 0;
  for (auto i = 0; i != n; i++) {
    int tp;
    cin >> tp;
    tot[cr] += (tp / 2);
    tot[1 - cr] += tp - (tp / 2);
    cr = 1 - cr;
  }
  cout << min(tot[0], tot[1]);
  return 0;
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int tc = 1;
  for (int i = 1; i <= tc; i++) esc(i);
}
