Positive integer $\underline{a}\,\, \underline{b}\,\, \underline{c}\,\, \underline{d}\,\, \underline{r}\,\, \underline{s}\,\, \underline{t}$  has digits $a$, $b$, $c$, $d$, $r$, $s$, and $t$, in that order, and none of the digits is $0$. The two-digit numbers $\underline{a}\,\, \underline{b}$ ,  $\underline{b}\,\, \underline{c}$ , $\underline{c}\,\, \underline{d}$ , and $\underline{d}\,\, \underline{r}$ , and the three-digit number $\underline{r}\,\, \underline{s}\,\, \underline{t}$ are all perfect squares. Find $\underline{a}\,\, \underline{b}\,\, \underline{c}\,\, \underline{d}\,\, \underline{r}\,\, \underline{s}\,\, \underline{t}$ .

Let's think step by step and output the final answer within \boxed{}.
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