  The Wentzel-Kramers-Brillouin (WKB) perturbative series, a widely used
technique for solving linear waves, is typically divergent and at best,
asymptotic, thus impeding predictions beyond the first few leading-order
effects. Here, we report a closed-form formula that exactly resums the
perturbative WKB series to all-orders for two turning point problem. The
formula is elegantly interpreted as the action evaluated using the product of
spatially-varying wavenumber and a coefficient related to the wave
transmissivity; unit transmissivity yields the Bohr-Sommerfeld quantization.
