#include <bits/stdc++.h>
using namespace std;
const int maxn = 3e5 + 10;
const int mod = 1e9 + 7;
const int inf = 0x3f3f3f3f;
const long long INF = 0x3f3f3f3f3f3f3f3f;
long long dp[maxn][3];
long long a[maxn];
long long cnt[2];
void solve() {
  int n;
  scanf("%d", &n);
  for (int i = 1; i <= n; i++) {
    scanf("%lld", &a[i]);
    cnt[i % 2] += a[i] / 2;
    cnt[(i + 1) % 2] += (a[i] + 1) / 2;
  }
  printf("%lld\n", min(cnt[1], cnt[0]));
}
int main() {
  clock_t t1 = clock();
  solve();
end:
  cerr << "Time used " << clock() - t1 << " ms" << endl;
  return 0;
}
