**Definitions**
Let $ n \in \mathbb{Z}^+ $ be the length of strings $ s $ and $ t $.
Let $ s, t \in \{a, b\}^n $ be two strings of equal length consisting of characters 'a' and 'b'.
**Constraints**
1. $ 1 \leq n \leq 2 \cdot 10^5 $
2. $ s, t $ contain only characters 'a' and 'b'.
**Objective**
Determine the minimum number of operations to make $ s = t $, where each operation consists of swapping $ s[i] $ with $ t[j] $ for any indices $ i, j \in \{1, \dots, n\} $.
Define the set of mismatched positions:
$$ D = \{ i \in \{1, \dots, n\} \mid s[i] \ne t[i] \} $$
For each $ i \in D $, define the pair $ (s[i], t[i]) \in \{ (a,b), (b,a) \} $.
Let:
- $ d_{ab} = |\{ i \in D \mid s[i] = a, t[i] = b \}| $
- $ d_{ba} = |\{ i \in D \mid s[i] = b, t[i] = a \}| $
**Feasibility Condition**
It is possible to make $ s = t $ if and only if $ d_{ab} + d_{ba} $ is even.
**Objective Function**
If feasible, the minimum number of operations is:
$$ k = \left\lceil \frac{d_{ab}}{2} \right\rceil + \left\lceil \frac{d_{ba}}{2} \right\rceil $$
**Output**
- If $ d_{ab} + d_{ba} $ is odd, output $-1$.
- Otherwise, output $ k $, followed by $ k $ pairs $ (i, j) $ representing swaps between $ s[i] $ and $ t[j] $ that achieve equality.