#include <bits/stdc++.h>
using namespace std;
long long int solve(int n) {
  long long zero = 0, one = 0, x;
  int a = 0;
  for (int i = 0; i < n; i++) {
    cin >> x;
    if (a) {
      zero += (x / 2);
      one += (x - (x / 2));
    } else {
      one += (x / 2);
      zero += (x - (x / 2));
    }
    a = !a;
  }
  return min(zero, one);
}
int main() {
  int n;
  cin >> n;
  cout << solve(n);
}
