class Stack:
    def __init__(self):
        self.stack = ""
        self.cache = []
        
    def append(self, value):
        self.cache.append(self.stack)
        self.stack += value
    
    def pop_k(self, k_value):
        self.cache.append(self.stack)
        if len(self.stack) < k_value:
            self.stack = ""
        else:
            self.stack = self.stack[:-k_value]
    
    def print_c(self, kth_value):
        if len(self.stack) < kth_value:
            return ""
        return self.stack[kth_value-1]
        
    def undo(self):
        self.stack = self.cache[-1]
        self.cache.pop()