#include <bits/stdc++.h>
using namespace std;
void solve() {
  int n;
  cin >> n;
  long long a = 0;
  long long b = 0;
  for (int i = 0; i < n; i++) {
    int x;
    cin >> x;
    if (x % 2 == 0) {
      a += x / 2;
      b += x / 2;
      continue;
    }
    a += x / 2;
    b += x / 2;
    if (i % 2 == 0)
      a++;
    else
      b++;
  }
  cout << min(a, b) << "\n";
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  solve();
}
