text: // O(sqrt(x)) Exhaustive Primality Test
  #include <cmath>
  #define EPS 1e-7
  typedef long long LL;
  bool IsPrimeSlow (LL x)
  {
    if(x<=1) return false;
    if(x<=3) return true;
    if (!(x%2) || !(x%3)) return false;
    LL s=(LL)(sqrt((double)(x))+EPS);
    for(LL i=5;i<=s;i+=6)
    {
      if (!(x%i) || !(x%(i+2))) return false;
    }
    return true;
  }
  // Primes less than 1000:
  //      2     3     5     7    11    13    17    19    23    29    31    37
  //     41    43    47    53    59    61    67    71    73    79    83    89
  //     97   101   103   107   109   113   127   131   137   139   149   151
  //    157   163   167   173   179   181   191   193   197   199   211   223
  //    227   229   233   239   241   251   257   263   269   271   277   281
  //    283   293   307   311   313   317   331   337   347   349   353   359
  //    367   373   379   383   389   397   401   409   419   421   431   433
  //    439   443   449   457   461   463   467   479   487   491   499   503
  //    509   521   523   541   547   557   563   569   571   577   587   593
  //    599   601   607   613   617   619   631   641   643   647   653   659
  //    661   673   677   683   691   701   709   719   727   733   739   743
  //    751   757   761   769   773   787   797   809   811   821   823   827
  //    829   839   853   857   859   863   877   881   883   887   907   911
  //    919   929   937   941   947   953   967   971   977   983   991   997
  
  // Other primes:
  //    The largest prime smaller than 10 is 7.
  //    The largest prime smaller than 100 is 97.
  //    The largest prime smaller than 1000 is 997.
  //    The largest prime smaller than 10000 is 9973.
  //    The largest prime smaller than 100000 is 99991.
  //    The largest prime smaller than 1000000 is 999983.
  //    The largest prime smaller than 10000000 is 9999991.
  //    The largest prime smaller than 100000000 is 99999989.
  //    The largest prime smaller than 1000000000 is 999999937.
  //    The largest prime smaller than 10000000000 is 9999999967.
  //    The largest prime smaller than 100000000000 is 99999999977.
  //    The largest prime smaller than 1000000000000 is 999999999989.
  //    The largest prime smaller than 10000000000000 is 9999999999971.
  //    The largest prime smaller than 100000000000000 is 99999999999973.
  //    The largest prime smaller than 1000000000000000 is 999999999999989.
  //    The largest prime smaller than 10000000000000000 is 9999999999999937.
  //    The largest prime smaller than 100000000000000000 is 99999999999999997.
  //    The largest prime smaller than 1000000000000000000 is 999999999999999989.
  