Stones

import java.io.*;

public class Main {

    public static void main(String[] argv) {

        IO io = new IO();

        char[] input = io.next().toCharArray();
        char prev = input[0];

        int ans = 0;

        for(int i = 1; i < input.length; i++) {
            if(input[i] != prev) {
                prev = input[i];
                continue;
            }
            ans++;
        }

        System.out.println(ans);
    }

}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: ACCEPTED

Test 15

Verdict: ACCEPTED