#include <bits/stdc++.h>
using namespace std;
int a[300005];
int main() {
  ios::sync_with_stdio(0);
  long long n, m, i, j, t, x, y, w, b;
  w = b = 0;
  cin >> n;
  for (i = 0; i < n; i++) {
    cin >> a[i];
  }
  for (i = 0; i < n; i++) {
    w += a[i] / 2;
    b += a[i] / 2;
    if (i % 2 == 0) {
      w += a[i] % 2;
    } else {
      b += a[i] % 2;
    }
  }
  cout << min(w, b);
  return 0;
}
