class IsContInt(object):
    def __init__(self):
        pass
    
    def constructor(self, data):
        data = self.str2int(data)
        # self.A = data[0]
        # self.B = data[1]
        # self.C = data[2]
        # self.d = data[3]
        data.sort()
        self.data = data
    def run(self):
        self.analysis()
    
    def analysis(self):
        
        for num1, num2 in zip(self.data, self.data[1:]):
            if num1==num2-1:
                continue
            else:
                print("No")
                exit()
        print("Yes")
        
    @staticmethod
    def str2int(data):
        data = map(int, data)
        return list(data)
    
    
if __name__=="__main__":
    data = input().split(" ")
    func = IsContInt()
    func.constructor(data=data)
    func.run()