#include <bits/stdc++.h>
const int maxn = 300000 + 10;
int a[maxn];
int main() {
  int n;
  scanf("%d", &n);
  for (int i = 0; i < n; i++) scanf("%d", a + i);
  long long black = 0, white = 0;
  for (int i = 0; i < n; i++) {
    if (i & 1) {
      int b = a[i] / 2;
      int w = a[i] - b;
      black += b;
      white += w;
    } else {
      int w = a[i] / 2;
      int b = a[i] - w;
      black += b;
      white += w;
    }
  }
  printf("%lld\n", black < white ? black : white);
  return 0;
}
