#include <bits/stdc++.h>
using namespace std;
const int MAXN = 1e6 + 5;
const int MOD = 998244353;
int main() {
  int n;
  while (cin >> n) {
    int p;
    long long x = 0, y = 0;
    for (int i = 1; i <= n; i++) {
      cin >> p;
      x += p / 2;
      y += p / 2;
      if (p % 2) {
        if (i % 2)
          x++;
        else
          y++;
      }
    }
    printf("%I64d\n", min(x, y));
  }
}
