#include <bits/stdc++.h>
using namespace std;
long long ans, curr, a1, flg, n;
int main() {
  cin >> n;
  for (int i = 1; i <= n; i++) {
    scanf("%d", &curr);
    ans += curr;
    if (flg)
      a1 += (curr + 1) / 2;
    else
      a1 += curr / 2;
    flg ^= 1;
  }
  cout << min(ans - a1, a1) << endl;
  return 0;
}
