#include <stdio.h>
#include <stdlib.h>
#include <math.h>

#define LIMIT 1000000000

int isPrime ( int n )
{
   int d, m;

   m = sqrt(n);
   for (d = 2; d <= m; ++d) {
      if (n % d == 0) return 0;
   }
   return 1;
}

int genLarger ( int n, int maxn )
{
   int m, N;

   m = maxn;
   if (isPrime(n)) {
      if (n > maxn) m = n;
      if (n < LIMIT / 10) {
         N = genLarger(10*n+1,m); if (N > m) m = N;
         N = genLarger(10*n+3,m); if (N > m) m = N;
         N = genLarger(10*n+7,m); if (N > m) m = N;
         N = genLarger(10*n+9,m); if (N > m) m = N;
      }
   }
   return m;
}

int main ()
{
   int m, maxn;

   maxn = 0;

   m = genLarger(2,maxn); if (m > maxn) maxn = m;
   m = genLarger(3,maxn); if (m > maxn) maxn = m;
   m = genLarger(5,maxn); if (m > maxn) maxn = m;
   m = genLarger(7,maxn); if (m > maxn) maxn = m;

   printf("Largest truncatable prime = %d\n", maxn);

   exit(0);
}