#include <bits/stdc++.h>
using namespace std;
inline long long read() {
  char ch = getchar();
  long long x = 0, f = 1;
  while (ch < '0' || ch > '9') {
    if (ch == '-') f = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9') x = x * 10 + (ch ^ 48), ch = getchar();
  return x * f;
}
int main() {
  long long n, a = 0, b = 0;
  n = read();
  for (long long i = 1; i <= n; i++) {
    long long m;
    m = read();
    a += m / 2;
    b += m / 2;
    if (i % 2) {
      b += m % 2;
    } else {
      a += m % 2;
    }
  }
  cout << min(a, b);
  return 0;
}
