#include <bits/stdc++.h>
using namespace std;
void solve();
int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  ;
  long long t = 1;
  while (t--) solve();
}
void solve() {
  long long n, a = 0, b = 0, x;
  cin >> n;
  for (long long i = 0; i < n; i++) {
    cin >> x;
    long long l = x / 2, r = x / 2;
    if (x % 2 == 1) l++;
    if (i % 2 == 0)
      a += l, b += r;
    else
      a += r, b += l;
  }
  cout << min(a, b) << '\n';
}
