include

include

include

using namespace std;

int main() { long long n; int x; cin>>n; x=0; for (int i=2;i<=sqrt(n);i++ ) { if (n%i==0) x=x+1; } if (x==0) cout<<"-1"; else{

for (int i=2;i<n;i++)
    {
        if (n%i==0)
        {
            cout<<n-i;
            break;
        }
    }
}
return 0;

}