Incompatible types: 'String' and 'Integer'

Question

firstData:=copy(ExcelApp.cells[controlPlace,2],10,4);
sti_FirstData:=StrToInt(copy(firstData,4,1));
sti_FirstData+1=sti_NextData;
思路大概就用这三行代码可以表达出来吧，为什么转换类型那编译不能通过呢？

Answer 1

sti_FirstData+1=sti_NextData; 
这是什么意思

Answer 2

sti_NextData := sti_FirstData + 1;            ???

Answer 3

firstData:=copy(ExcelApp.cells[controlPlace,2],10,4); 
sti_FirstData:=StrToInt(copy(firstData,4,1)); //语法没错
sti_FirstData+1=sti_NextData; //没有这样的语法，而且也不能用=号，要用 ：=