#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  long long ans = 0;
  long long a[n + 5];
  for (int i = 1; i <= n; i++) cin >> a[i];
  long long black = 0, white = 0;
  int coin = 0;
  for (int i = 1; i <= n; i++) {
    if (coin == 0) {
      white += ((a[i] + 1) / 2);
      black += (a[i] / 2);
      coin = 1 - coin;
    } else {
      black += ((a[i] + 1) / 2);
      white += (a[i] / 2);
      coin = 1 - coin;
    }
  }
  cout << min(black, white) << endl;
}
