#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long int n;
  cin >> n;
  long long int i;
  long long int count = 0;
  long long int sum = 0;
  long long int c1 = 0;
  long long int a[n + 5];
  for (i = 0; i < n; i++) {
    cin >> a[i];
    sum += a[i];
    if (count == 0) {
      c1 += (a[i] + 1) / 2;
    } else {
      c1 += (a[i] / 2);
    }
    count++;
    count = count % 2;
  }
  long long int ans = (c1 < sum - c1 ? c1 : sum - c1);
  cout << ans << endl;
}
