= Solution
Alice adjoins $|0\rangle_{A'}$ and applies a local unitary whose action on the relevant inputs is
$$
|0\rangle|0\rangle\mapsto
\frac st|0\rangle|0\rangle+sqrt{1-\frac{s^2}{t^2}}|1\rangle|0\rangle,
\qquad
|0\rangle|1\rangle\mapsto|0\rangle|1\rangle.
$$
Measuring $A'$ and obtaining zero leaves the unnormalized state
$$
s(|00\rangle+|11\rangle),
$$
so the normalized state is the <Bell state> $|\phi^+\rangle$. The success probability is $2s^2>0$.
Applying this <entanglement concentration> independently to $n$ pairs yields an expected $2s^2n$ Bell pairs. Alice and Bob measure successful pairs in the computational basis to obtain identical secret random bits; public communication identifies successful positions without revealing outcomes. These bits form a secret key for a one-time pad of expected length $2s^2n$.
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