#include <bits/stdc++.h>
#pragma GCC optimize("O3")
using namespace std;
long long n, nr1, nr2, a[300002];
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  cin >> n;
  for (int i = 1; i <= n; i++) {
    cin >> a[i];
    if (i % 2) {
      nr1 += (a[i] + 1) / 2;
      nr2 += a[i] / 2;
    } else {
      nr1 += a[i] / 2;
      nr2 += (a[i] + 1) / 2;
    }
  }
  cout << min(nr1, nr2);
  return 0;
}
