#include <bits/stdc++.h>
using namespace std;
const int N = 3e5 + 10;
int n, a[N], hole[N], type[N];
int main() {
  cin >> n;
  for (int i = 1; i <= n; ++i) cin >> a[i];
  long long ans = 0;
  hole[0] = type[0] = 0;
  for (int i = 1; i <= n; ++i)
    if (hole[i - 1] == 0) {
      if (a[i] % 2 == 0) {
        ans += a[i] / 2;
        hole[i] = 0;
      } else {
        ans += a[i] / 2;
        hole[i] = 1;
        type[i] = 1;
      }
    } else if (type[i - 1] == 1) {
      type[i] = 0;
      ans += (a[i] + 1) / 2;
      if ((a[i] + 1) / 2 <= hole[i - 1])
        hole[i] = (a[i] % 2 == 1 ? ((a[i] + 1) / 2 - 1) : (a[i] + 1) / 2);
      else
        hole[i] = (a[i] % 2 == 1 ? hole[i - 1] - 1 : hole[i - 1]);
    } else {
      type[i] = 1;
      ans += a[i] / 2;
      if (a[i] / 2 <= hole[i - 1])
        hole[i] = (a[i] % 2 == 1 ? a[i] / 2 + 1 : a[i] / 2);
      else
        hole[i] = (a[i] % 2 == 1 ? hole[i - 1] + 1 : hole[i - 1]);
    }
  cout << ans << endl;
}
