function firstDifferenceIndex(string1, string2) {
	  var minLen = Math.min(string1.length, string2.length);
	  for (var i = 0; i < minLen; i++) {
	    if (string1.charAt(i) !== string2.charAt(i)) {
	      return i;
	    }
	  }
	  return string1.length === string2.length ? -1 : minLen;
	}
Finds the index of the first character
that's not common between the two given strings.

@return {number} the index of the character where the strings diverge