#include <bits/stdc++.h>
using namespace std;
 
// why am I so weak
 
int n;
int a[2555];
int dp[2555];
 
int main() {
	scanf("%d", &n);
 
	for (int i = 1; i <= n; i++) {
		scanf("%d", &a[i]);	
	}
 
	sort(a + 1, a + n + 1);
 
	memset(dp, 63, sizeof(dp));
 
	dp[0] = 0;
	dp[1] = 1;
 
	for (int i = 2; i <= n; i++) {
		if (abs(a[i] - a[i - 1]) <= 2) {
			dp[i] = min(dp[i], dp[i - 2] + 1);
		}
 
		dp[i] = min(dp[i], dp[i - 1] + 1);
	}
 
	printf("%d\n", dp[n]);
 
	return 0;
}

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