백준 C++ 2839번

#include 

using namespace std;

int main(void)
{
	int N,a,b,c;
	
	cin >> N;
	if(N<3 || N > 5000)
		return -1;
	a = N/5;
	//cout << a << ' ';
	b = (N-a*5)/3;
	//cout << b << ' ';
	c = N-a*5-b*3;
	//cout << c << endl;
	while(c!=0 && a>0){
	static int count = 1;
	//cout << count;
	if(c==1 || c==2){
	a = (N-5*count)/5 ;
	//cout << a << ' ';
	b = (N-a*5)/3;
	//cout << b << ' ';
	c = (N-a*5)-b*3;
	//cout << c << endl;
	count++;
	
	}
	else break;
	}
	

	if(c!=0){
		cout << -1;
		return -1;
	}
	
	cout << a+b;
	return 0;
						
}