#include <bits/stdc++.h>
using namespace std;
template <typename elemType>
inline void Read(elemType &T) {
  elemType X = 0, w = 0;
  char ch = 0;
  while (!isdigit(ch)) {
    w |= ch == '-';
    ch = getchar();
  }
  while (isdigit(ch)) X = (X << 3) + (X << 1) + (ch ^ 48), ch = getchar();
  T = (w ? -X : X);
}
long long Data[300010];
int N;
int main() {
  Read(N);
  long long A = 0, B = 0;
  for (register int i = 1; i <= N; ++i) {
    Read(Data[i]);
    if (i & 1) {
      A += Data[i] >> 1;
      B += Data[i] - (Data[i] >> 1);
    } else {
      A += Data[i] - (Data[i] >> 1);
      B += Data[i] >> 1;
    }
  }
  printf("%I64d\n", min(A, B));
  return 0;
}
