check if the letter in string1 is present in string2
if not then append that letter in uncommon1 array
check if the letter in string2 is present in string1
if not then append that letter in uncommon2 array
uncommon array is the result of concatenation of uncommon1 array and uncommon2 array
if length of uncommon array is zero than
print-1
else print the uncommon array as a string using join
