public static int alternatingCharacters(String str) {
    int firstptr=0;
    int secondptr = firstptr+1;
    int totaldeletions = 0;
    while(secondptr < str.length()) {
        if(str.charAt(firstptr) == str.charAt(secondptr)) {
            totaldeletions++;
            secondptr++;
        } else {
            firstptr = secondptr;
            secondptr++;
        }
    }
    return totaldeletions;        
}