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