Add Two String Numbers

Write a function that adds two numbers. The catch, however, is that the numbers will be strings.

Examples

addStrNums("4", "5") ➞ "9"

addStrNums("abcdefg", "3") ➞ "-1"

addStrNums("1", "") ➞ "1"

addStrNums("1874682736267235927359283579235789257", "32652983572985729") ➞ "1874682736267235927391936562808774986"

Notes

• If there are any non-numerical characters, return "-1".
• An empty parameter should be treated as "0".
• Your function should be able to add any size number.
• Your function doesn't have to add negative numbers.
• Zeros at the beginning of the string should be trimmed.