#include <bits/stdc++.h>
using namespace std;
const int N = 100050;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  unsigned long long black = 0, white = 0, n, i, a;
  cin >> n;
  for (i = 0; i < n; ++i) {
    cin >> a;
    if (i % 2 == 0) {
      black += a / 2;
      white += a - a / 2;
    } else {
      white += a / 2;
      black += a - a / 2;
    }
  }
  cout << min(black, white);
  return 0;
}
