PatratMagic4

#include <cmath>
long long int magic(unsigned long long int c)
{
    if(c<15)
        return 0;
    double C=(double)c,u,s,t,d=2*sqrt(C*C+1/27);
    long long n=0;
    if(d==0)
        u=-C,t=cbrt(u),s=cbrt(2*C-u);
    if(d>0)
        u=(-C+sqrt(C*C+1/27)),t=cbrt(u),s=cbrt(2*C+u);
    n=(long long int)(s-t);
    if(c==1ULL*(n*n*n+n)/2)
        return n;
    return 0;
}