On a calculator, if only numbers 7, 0 and + and = keys can be used, then in order to display "222222", how many times must the number "7" be pressed at least.
在一台计算器上，如果只能使用数字7，0及+和=键，那么为了显示"222222"，最少要按数字"7"键多少次．
<result>21</result>