#include <bits/stdc++.h>
using namespace std;
const int inf = 1e9 + 7;
const long long longinf = 1LL << 60;
const long long mod = 1e9 + 7;
const long double eps = 1e-10;
template <typename T1, typename T2>
inline void chmin(T1 &a, T2 b) {
  if (a > b) a = b;
}
template <typename T1, typename T2>
inline void chmax(T1 &a, T2 b) {
  if (a < b) a = b;
}
void solve() {
  int n;
  cin >> n;
  vector<long long> A(n);
  for (int i = (int)(0); i < (int)(n); i++) cin >> A[i];
  vector<long long> cnt(2);
  for (int i = (int)(0); i < (int)(n); i++) {
    cnt[i & 1] += A[i] / 2;
    cnt[!(i & 1)] += A[i] - A[i] / 2;
  }
  cout << min(cnt[0], cnt[1]) << '\n';
}
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  int T = 1;
  while (T--) solve();
  return 0;
}
