#include <bits/stdc++.h>
using namespace std;
const long long MAX = 1e6 + 100;
const long long mod = 1e9 + 7;
const long long inf = 1e18 + 12;
vector<long long> arr(MAX);
map<long long, long long> mm;
int main() {
  ios_base::sync_with_stdio(false);
  cout.tie(NULL);
  cout.tie(NULL);
  ;
  long long tt = 1;
  while (tt--) {
    long long n;
    cin >> n;
    long long a[n];
    for (long long i = 0; i < n; i++) cin >> a[i];
    long long w = 0, b = 0;
    long long i = 0;
    while (i < n) {
      if (i % 2 == 0) {
        w += (a[i] + 1) / 2;
        b += (a[i]) / 2;
      } else {
        w += a[i] / 2;
        b += (a[i] + 1) / 2;
      }
      i++;
    }
    cout << min(w, b);
  }
}
