Choose the missing terms out of the given alternatives Z, X, S, I, R, R, ?, ?

Z - 2 = X
X - 5 = S
S - 10 = I
I - 17 = R
R - 26 = R
R - 37 = G
G - 50 = I
The series 2, 5, 10, 17, 26, 37, 50 is obtained by the pattern +3, +5, +7, +9, . . . .