#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, x;
  scanf("%d", &n);
  long long black = 0, white = 0;
  for (int i = 0; i < n; i++) {
    scanf("%d", &x);
    white += x / 2;
    black += x / 2;
    if (x & 1) {
      if (i % 2)
        white++;
      else
        black++;
    }
  }
  printf("%lld", min(white, black));
}
