PAT B1007 Prime number pair conjecture (20 points)

#include <cstdio>
#include <cmath>
#include <cstdlib>

int main(){
    
    
	int n;
	scanf("%d", &n);
	
	int table[n];
	int num = 0;
	bool isPrime[n] = {
    
    };
	
	for(int i=2; i<=n; i++){
    
    
		if(!isPrime[i]){
    
    
			table[num] = i;
			num++;
			for(int j=i+i; j<=n; j+=i){
    
    
				isPrime[j] = true;
			}
		}
	}
	
	int sum = 0;
	for(int i=0; i<num-1; i++){
    
    
		if(abs(table[i]-table[i+1]) == 2){
    
    
			sum++;
		}
	}
	printf("%d", sum);
		
	return 0;
}