#include <bits/stdc++.h>
using namespace std;
long long n;
const long long N = 3e5 + 10;
signed main() {
  cin >> n;
  long long black = 0, white = 0;
  for (long long i = 1; i <= n; i++) {
    long long x;
    cin >> x;
    black += x / 2;
    white += x / 2;
    if (x & 1) {
      if (i & 1)
        black++;
      else
        white++;
    }
  }
  cout << min(black, white) << endl;
  return 0;
}
