#include <bits/stdc++.h>
#pragma comment(linker, "/STACK:36777216")
using namespace std;
long long mod = 998244353;
long long inf = 1e9;
long double eps = 1e-7;
long double pi = acosl(-1);
ifstream in("input.txt");
ofstream out("output.txt");
int main() {
  int n;
  cin >> n;
  long long b = 0, w = 0;
  for (int i = 0; i < n; i++) {
    int t;
    cin >> t;
    if (i % 2 == 0) {
      b += t / 2;
      w += t - t / 2;
    } else {
      w += t / 2;
      b += t - t / 2;
    }
  }
  cout << min(w, b);
  return 0;
}
