Suppose we have a B$-$Tree $($NOT the one shown below$)$ that holds keys and leaves. There are $200$ million data records $($N$),$ and they are stored in the leaves $($lowest level$)$ of the B$-$Tree. Each leaf contains $100$ data records. The leaves are stored only at the bottom level of the tree $($below the keys$),$ and each key at the bottom level of keys points to one leaf. The B$-$Tree has $4$ levels of keys counting the root level which also has $M$ keys. Except for the bottom level, each key has M child keys. The bottom level keys have leaves $($one leaf per key$).$