Exactly 10 of all simple Lie superalgebras of vector fields on 1|N -dimensional supercircles (superstrings) preserving a structure (either nothing, or a volume element, or a contact form) have nontrivial central extensions. The values of the central charges in (projective) spinor-oscillator representations of these stringy superalgebras associated with the adjoint module can be interpreted as critical dimensions of respective superstrings. Apart from the well-known values 26, 10, 2 and 0 corresponding to N = 0, 1, 2 and > 2, respectively, for the contact type stringy superalgebras (Neveu-Schwarz and Ramond types alike), there are two more non-zero values of critical dimension: For the general and divergence-free algebras for N = 2. These dimensions, found here, are −1 and −2, respectively. We also mention related problems.