#include <bits/stdc++.h>
const double eps = (1e-9);
using namespace std;
int dcmp(long double a, long double b) {
  return fabsl(a - b) <= eps ? 0 : (a > b) ? 1 : -1;
}
int getBit(long long num, int idx) { return ((num >> idx) & 1ll) == 1; }
int setBit1(long long num, long long idx) { return num | (1ll << idx); }
long long setBit0(long long num, int idx) { return num & ~(1ll << idx); }
long long flipBit(long long num, int idx) { return num ^ (1ll << idx); }
void run() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
}
int countNumBit1(int mask) {
  int ret = 0;
  while (mask) {
    mask &= (mask - 1);
    ++ret;
  }
  return ret;
}
const int N = 300000 + 9;
long long arr[N];
int main() {
  run();
  int n;
  cin >> n;
  long long wh = 0, bl = 0;
  for (int i = 1; i <= n; i++) cin >> arr[i];
  int mx = arr[1];
  for (int i = 1; i <= n; i++) {
    int no;
    no = arr[i];
    if (i % 2 == (mx - no + 1) % 2) {
      wh += (no + 1) / 2;
      bl += no / 2;
    } else {
      wh += no / 2;
      bl += (no + 1) / 2;
    }
  }
  cout << min(wh, bl) << endl;
}
