func marsExploration(s: String) -> Int {
    var changed = 0
    for (index, c) in s.enumerated() {
        switch index % 3 {
            case 0, 2:
               if c != "S" {
                    changed = changed + 1
               }
            case 1:
                if c != "O" {
                    changed = changed + 1
               }
            default:
                break
        }
    }
    return changed
}