#include <bits/stdc++.h>
#pragma GCC optimize("Ofast", "unroll-loops", "omit-frame-pointer", "inline")
#pragma GCC option("arch=native", "tune=native", "no-zero-upper")
#pragma GCC target("avx2")
#pragma GCC optimize("03")
#pragma GCC target("sse4")
using namespace std;
int a[(long long)(5e5 + 5)];
int a2[(long long)(5e5 + 5)];
int32_t main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  int n;
  cin >> n;
  long long s1 = 0;
  long long s2 = 0;
  for (int i = 0; i < n; i++) {
    cin >> a[i];
    if (i & 1) {
      s1 += a[i] / 2;
      s2 += a[i] - (a[i] / 2);
    } else {
      s2 += a[i] / 2;
      s1 += a[i] - (a[i] / 2);
    }
  }
  cout << (min(s1, s2)) << '\n';
  return 0;
}
