#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int n;
  cin >> n;
  long long int white = 0, black = 0, flag = 0;
  for (int i = 1; i <= n; i++) {
    long long int x;
    cin >> x;
    if (flag == 0) {
      black += x / 2;
      white += x - x / 2;
    } else {
      white += x / 2;
      black += x - x / 2;
    }
    flag = 1 - flag;
  }
  cout << min(white, black) << "\n";
  return 0;
}
