#include <bits/stdc++.h>
using namespace std;
long long int insig;
void Out(vector<long long int> x) {
  for (long long int i = 0; i < x.size(); i++) {
    cout << x[i] << " ";
  }
  cout << endl;
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long int N;
  cin >> N;
  vector<long long int> a;
  for (long long int IBRO = 0; IBRO < N; IBRO++) {
    cin >> insig;
    a.push_back(insig);
  };
  long long int black = 0;
  long long int white = 0;
  for (long long int i = 0; i < N; i++) {
    if (i % 2 == 0) {
      black += ((a[i] + 1) / 2);
      white += (a[i] / 2);
    } else {
      black += ((a[i]) / 2);
      white += ((a[i] + 1) / 2);
    }
  }
  cout << min(black, white) << endl;
  return 0;
}
