first page number: {{#var: varFirstPageNumber}}
{{#iferror:{{#ifexpr:{{#var: varFirstPageNumber}}^0 = 1 | number | string }}| string }}
{{#if: {{IsNumber|value={{var: varFirstPageNumber}}|}}<!--
-->|<!--
--><!-- regular number: store as is
-->{{#set:First Page={{#var: varFirstPageNumber}}|}}<!--
--><!--
-->|<!--
--><!-- not a number: see if it is roman
-->{{#set:First Page=0|}}<!--
--><!--
-->}}
roman: {{#roman: {{#var: varFirstPageNumber}}}}