#include <bits/stdc++.h>
using namespace std;
const int N = 2e5 + 2;
const int M = 1e5 + 2;
const int mod = 998244353;
const int inf = 1e9;
const long long INF = 1e16;
void data() {}
template <typename T>
void scerr(T nums) {
  for (auto it : nums) cerr << it << " | ";
  cerr << "\n";
}
long long n, a, cntA, cntB;
int main() {
  data();
  scanf("%lld", &n);
  for (int i = 1; i <= n; ++i) {
    scanf("%d", &a);
    if (i & 1) {
      cntA += (a / 2) + (a % 2);
      cntB += (a / 2);
    } else {
      cntB += (a / 2) + (a % 2);
      cntA += (a / 2);
    }
  }
  printf("%lld", min(cntA, cntB));
}
