#include<cstdio>
#include<cstring>
using namespace std;
const int N=1e6+5;
int a[N];

int main(){
	int n;
	int val,ans;
	while(~scanf("%d",&n)){
		ans=0;
		memset(a,0,sizeof(a));
		for(int i=1;i<=n;i++){
			scanf("%d",&val);
			a[val]++;
		}
		
		for(int i=1;i<=n;i++){
			ans+=a[i]/2;  //对子优先 
			a[i]%=2;
			
			if(a[i]&&a[i+1]%2&&a[i+2]&&i+1<n){  //第三个数顺子优先 
				ans++;
				a[i]--;
				a[i+1]--;
				a[i+2]--;
			}
		}
		printf("%d\n",ans);
	}
	return 0;
}