#include<stdio.h>
int main(){
  int i,number,serial;
  scanf("%d", &number);
  long long answer=0;

  for(i=1;i<=number;i++){
    scanf("%d", &serial);
  }
  for(i=1;i<=number;i++){
    if (i%2==0){
    answer=answer+i;
    }
  }
  printf("%lld\n", answer);


  return 0;
}
