Given a sequence of one or more consecutive natural numbers concatenated into a string, return the smallest possible
first number in the sequence.
Numbers will never be truncated.
"123" -> [1, 2, 3] -> 1
"91011" -> [9, 10, 11] -> 9
"17181920" -> [17, 18, 19, 20] -> 17
"9899100" -> [98, 99, 100] -> 98
"121122123" -> [121, 122, 123] -> 121
"1235" -> [1235] -> 1235
"101" -> [101] -> 101
0 < length string < 140
0 < smallest number < 1 000 000 000