Connectibles

#include <stdio.h>
#include <gmp.h>

int main(void)
{
    mpz_t x;
    mpz_init(x);
    mpz_set_ui(x,18);
    int i;
    for(i=0;i<50;++i){
        mpz_mul_2exp(x,x,1);mpz_sub_ui(x,x,1);//x->2x-1
        int last = 0;
        while(1){//BEWARE!
            int current=mpz_probab_prime_p(x,30);//The probability of a pseudoprime is (1/4)^30
            if(current&&last)break;//Miller-Rabin probabilistic test
            last=current;
            mpz_sub_ui(x,x,2);
        }
        mpz_add_ui(x,x,1);//x and x+2 are primes,so we set x->x+1
    }
    gmp_printf("%Zd\n",x);
    mpz_clear(x);
    return 0;
}