#include <bits/stdc++.h>
using namespace std;
const long long mod = 1e9 + 7;
const int maxn = 1e5 + 9;
const int inf = 0x3f3f3f3f;
long long rd() {
  long long ans = 0;
  char last = ' ', ch = getchar();
  while (!(ch >= '0' && ch <= '9')) last = ch, ch = getchar();
  while (ch >= '0' && ch <= '9') ans = ans * 10 + ch - '0', ch = getchar();
  if (last == '-') ans = -ans;
  return ans;
}
int main() {
  int n = rd();
  long long x = 0, y = 0;
  int f = 0;
  while (n--) {
    int p = rd();
    if (f)
      x += p / 2, y += (p + 1) / 2;
    else
      y += p / 2, x += (p + 1) / 2;
    f = !f;
  }
  printf("%lld\n", min(x, y));
  return 0;
}
