#include <bits/stdc++.h>
using namespace std;
int power(int a, int b, int m, int ans = 1) {
  for (; b; b >>= 1, a = 1LL * a * a % m)
    if (b & 1) ans = 1LL * ans * a % m;
  return ans;
}
int main() {
  int n, first;
  long long c0 = 0, c1 = 0;
  cin >> n;
  for (int i = 1; i <= n; i++) {
    cin >> first;
    c0 += first / 2;
    c1 += first / 2;
    if (first & 1 and i & 1) c0++;
    if (first & 1 and !(i & 1)) c1++;
  }
  cout << min(c0, c1) << endl;
  return 0;
}
