------
## 最终结论
```python
def minimum_swaps_to_make_equal(s1, s2):
    xy_count = 0
    yx_count = 0
    
    for char1, char2 in zip(s1, s2):
        if char1 == 'x' and char2 == 'y':
            xy_count += 1
        elif char1 == 'y' and char2 == 'x':
            yx_count += 1
            
    # If the total number of mismatches is odd, return -1 since it's impossible
    if (xy_count + yx_count) % 2 != 0:
        return -1
    
    # Each pair of 'xy' and 'yx' can be resolved with one swap
    swaps = (xy_count // 2) + (yx_count // 2)
    
    # If there is an odd number of mismatches for both 'xy' and 'yx'
    if xy_count % 2 == 1 and yx_count % 2 == 1:
        # One additional swap is needed for the remaining mismatch
        swaps += 2
    
    return swaps

# Example usage:
s1 = "xx"
s2 = "yy"
print(minimum_swaps_to_make_equal(s1, s2))  # Output: 1

s1 = "xy"
s2 = "yx"
print(minimum_swaps_to_make_equal(s1, s2))  # Output: 2

s1 = "xx"
s2 = "xy"
print(minimum_swaps_to_make_equal(s1, s2))  # Output: -1

s1 = "xxyyxyxyxx"
s2 = "xyyxyxxxyx"
print(minimum_swaps_to_make_equal(s1, s2))  # Output: 4
```