#include <iostream>
using namespace std;
int main()
{
int fivB, thrB = 0;
int n;
cin >> n;
for(int i = n; i > 0;){
if(i < 3 || i == 4){
fivB = -1;
break;
}
else{
if(i%3 == 0 && i < 15){
i -= 3;
thrB++;
}
else{
i -= 5;
fivB++;
}
}
}
cout << fivB + thrB;
}
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