#include <bits/stdc++.h>
using namespace std;
long long dp[1000000][2];
const long long MOD = 1e9 + 7;
int main() {
  long long n, bl = 0, wh = 0;
  cin >> n;
  for (int i = 0; i < n; i++) {
    long long num;
    cin >> num;
    if (i % 2 == 0) {
      bl += num / 2;
      wh += (num - 1) / 2 + 1;
    } else {
      wh += num / 2;
      bl += (num - 1) / 2 + 1;
    }
  }
  cout << min(wh, bl);
  return 0;
}
