#include <bits/stdc++.h>
using namespace std;
inline long long read() {
  char ch = getchar();
  long long x = 0, f = 1;
  while (ch < '0' || ch > '9') {
    if (ch == '-') f = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9') {
    x = (x << 1) + (x << 3) + (ch ^ 48);
    ch = getchar();
  }
  return x * f;
}
const long long maxn = 3e5 + 10;
long long n;
long long a[maxn];
long long dp[maxn];
int main(void) {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  n = read();
  for (int i = 1; i <= n; ++i) a[i] = read();
  long long ans = 0;
  long long t1 = 0, t2 = 0;
  for (int i = 1; i <= n; ++i) {
    ans += a[i] / 2;
    if (a[i] & 1) {
      if (i % 2 == 0)
        t1++;
      else
        t2++;
    }
  }
  ans += min(t1, t2);
  cout << ans << endl;
  return 0;
}
