#include <bits/stdc++.h>
using namespace std;
int32_t main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  long long n;
  cin >> n;
  vector<long long> v(n);
  for (long long i = 0; i < n; i++) {
    cin >> v[i];
  }
  long long black = 0, white = 0;
  for (long long i = 0; i < n; i++) {
    long long x = v[i] / 2, y = v[i] - x;
    if (i & 1) {
      black += x;
      white += y;
    } else {
      white += x;
      black += y;
    }
  }
  cout << min(black, white);
}
