public static int marsExploration(String str) {
    int count = 0;
    for(int i=1; i<=str.length();i++) {
        char ch = str.charAt(i-1);
        int loc = i%3;
        if((loc == 1 && ch != 'S') || 
            (loc == 2 && ch != 'O') || 
            (loc == 0 && ch != 'S')) 
        count++;
    }
    return count;
}