#include <bits/stdc++.h>
using namespace std;
const int mod = 1000000007;
int n, a[300010];
int main() {
  ios_base::sync_with_stdio(false);
  cin >> n;
  for (int i = 1; i <= n; ++i) cin >> a[i];
  long long ans = 0;
  int pre = 0, odd = 0;
  for (int i = 1; i <= n; ++i) {
    int j = i;
    while (j + 1 <= n && a[j + 1] == a[i]) j++;
    int L = j - i + 1;
    if (L & 1) {
      if (pre == 0) {
        ans += 1LL * a[i] * L / 2;
        if (a[i] & 1) {
          pre = 1;
          odd = 1;
        }
      } else {
        int top = 0;
        if (odd)
          top = 2 * pre - 1;
        else
          top = 2 * pre;
        while (top > a[i]) {
          top -= 2;
          pre--;
        }
        while (a[i] - 2 >= top) {
          ans += 1LL * L;
          a[i] -= 2;
        }
        ans += 1LL * a[i] * (L / 2) + pre;
        if (odd) {
          if (top == a[i]) pre--;
        } else {
          if (top < a[i]) pre++;
        }
        odd ^= 1;
      }
    } else {
      ans += 1LL * a[i] * L / 2;
    }
    i = j;
  }
  cout << ans << endl;
  return 0;
}
