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.