#include <bits/stdc++.h>
using namespace std;
void program();
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  program();
}
const int MX = 1e6;
int n, x;
int a[MX];
void program() {
  cin >> n;
  for (int i = int(0); i < int(n); i++) cin >> a[i];
  long long ans1 = 0, ans2 = 0;
  for (int i = int(0); i < int(n); i++) {
    if (i % 2) {
      ans1 += a[i] / 2;
      ans2 += (a[i] + 1) / 2;
    } else {
      ans2 += a[i] / 2;
      ans1 += (a[i] + 1) / 2;
    }
  }
  cout << min(ans1, ans2) << endl;
}
