Percentage yield and purity

A sample of impure magnesium carbonate has a mass of 4.00 g. On heating, it decomposes completely to give 1.76 g of magnesium oxide as the only solid product.
$MgCO_3 \rightarrow MgO + CO_2$
What is the percentage purity of magnesium carbonate in the sample? (Ar: Mg = 24, C = 12, O = 16)