#include <bits/stdc++.h>
using namespace std;
const int N = 500005, mod = 1000000007;
long long int a, b, c, d, e, f, g, h[N], arr[N];
string s;
vector<long long int> v;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  cin >> a;
  long long int ans = 0, tek = 0, cift = 0;
  for (long long int i = 1; i <= a; i++) {
    cin >> arr[i];
    ans += arr[i] / 2;
    if (i % 2 == 0)
      cift += arr[i] % 2;
    else
      tek += arr[i] % 2;
    if (tek and cift) tek--, cift--, ans++;
  }
  cout << ans << endl;
}
