One mole of methyl 3,4-dihydroxybutanoate, $\mathrm{HOCH}{}_2-\mathrm{CH}(\mathrm{OH})-\mathrm{CH}{}_2-\mathrm{COOCH}{}_3$, is treated with a large excess of $\mathrm{CH}{}_3\mathrm{MgBr}$ in dry ether, followed by $\mathrm{H}{}_3\mathrm{O}{}^{+}$. Assume that each acidic $\mathrm{O}-\mathrm{H}$ consumes one mole of Grignard reagent (releasing $\mathrm{CH}{}_4$), that the ester is converted completely to a tertiary alcohol, and that no other reaction occurs. How many moles of $\mathrm{CH}{}_3\mathrm{MgBr}$ are consumed per mole of the compound?

Numerical answer type, tolerance 0.5.