#include <bits/stdc++.h>
using namespace std;
const int inf = 1e9, mod = 1e9 + 7;
int n, sl, fh;
long long cnt[2];
int rd() {
  sl = 0;
  fh = 1;
  char ch = getchar();
  while (ch < '0' || '9' < ch) {
    if (ch == '-') fh = -1;
    ch = getchar();
  }
  while ('0' <= ch && ch <= '9') sl = sl * 10 + ch - '0', ch = getchar();
  return sl * fh;
}
int main() {
  n = rd();
  for (int x, i = 1; i <= n; ++i) {
    x = rd();
    cnt[i & 1] += x / 2;
    cnt[(i + 1) & 1] += (x + 1) / 2;
  }
  printf("%lld\n", min(cnt[0], cnt[1]));
  return 0;
}
