#include <bits/stdc++.h>
using namespace std;
const long long mod = 1e9 + 7, N = 4e5 + 10;
void solve() {
  long long n;
  cin >> n;
  long long a[n];
  for (long long i = 0; i < n; i++) cin >> a[i];
  long long bl = 0, wh = 0;
  for (long long i = 0; i < n; i++) {
    if (i % 2 == 0) {
      long long t = a[i] / 2;
      long long t2 = a[i] - t;
      bl = bl + t2;
      wh = wh + t;
    } else {
      long long t = a[i] / 2;
      long long t2 = a[i] - t;
      bl = bl + t;
      wh = wh + t2;
    }
  }
  cout << min(bl, wh) << "\n";
  return;
}
int32_t main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long test = 1;
  while (test--) {
    solve();
  }
}
