#include <bits/stdc++.h>
using namespace std;
int main(void) {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int n;
  cin >> n;
  long long white = 0, black = 0;
  for (int i = 1; i <= n; ++i) {
    long long u;
    cin >> u;
    white += u / 2;
    black += u / 2;
    if (u & 1)
      if (i & 1)
        ++black;
      else
        ++white;
  }
  cout << min(black, white) << endl;
  return 0;
}
