#include <bits/stdc++.h>
#pragma GCC optimize("Ofast,no-stack-protector,unroll-loops,fast-math,O3")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
using namespace std;
long long sz, a, ca, cb;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  cout.precision(30);
  cerr.precision(7);
  ;
  cin >> sz;
  for (int i = 1; i <= sz; i++) {
    cin >> a;
    ca += (i % 2 ? (a / 2) + (a % 2) : (a / 2));
    cb += (i % 2 ? (a / 2) : (a / 2) + (a % 2));
  }
  cout << min(ca, cb) << "\n";
  return 0;
}
