#include <bits/stdc++.h>
using namespace std;
const int N = 300004;
int n, m;
long long a[N], f[N][4];
int main() {
  scanf("%d", &n);
  long long s = 0, t = 0, z;
  for (int i = 1; i <= n; i++) {
    cin >> z;
    s += z;
    if (i & 1)
      t += z / 2;
    else
      t += z - z / 2;
  }
  printf("%lld\n", min(t, s - t));
}
