#include <bits/stdc++.h>
using namespace std;
const long long maxn = 300086;
long long n, a[maxn];
inline long long min(long long x, long long y) { return x > y ? y : x; }
inline long long max(long long x, long long y) { return x > y ? x : y; }
inline long long read() {
  char ch = getchar();
  while (ch < 48) ch = getchar();
  long long r = 0;
  while (ch > 47) r = r * 10 + (ch ^ 48), ch = getchar();
  return r;
}
long long ans;
long long black, white;
signed main() {
  n = read();
  for (long long i = 1; i <= n; ++i) a[i] = read();
  for (long long i = 1; i <= n; ++i)
    if (i & 1)
      black += (a[i] >> 1), white += ((a[i] + 1) >> 1);
    else
      black += ((a[i] + 1) >> 1), white += ((a[i]) >> 1);
  cout << min(white, black);
}
