#include <bits/stdc++.h>
using namespace std;
const int MAXN = 700;
const int INF = 2e9;
void solve() {
  int n;
  cin >> n;
  long long int sum = 0, cnt = 0;
  for (int i = 0; i < n; ++i) {
    int x;
    cin >> x;
    if (i & 1)
      cnt += x / 2;
    else
      cnt += x / 2 + (x & 1);
    sum += x;
  }
  cout << min(cnt, sum - cnt);
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int t = 1;
  while (t--) {
    solve();
  }
  return 0;
}
