Truncatable primes
The number 3797 has an interesting property. Being prime itself, it is
possible to continuously remove digits from left to right, and remain
prime at each stage: 3797, 797, 97, and 7. Similarly we can work from
right to left: 3797, 379, 37, and 3.

Find the sum of the only eleven primes that are both truncatable from
left to right and right to left.

NOTE: 2, 3, 5, and 7 are not considered to be truncatable primes.