就是如下语句中的of
total=sum(of a1-a10);

我现在有10个宏变量，var1到var10，也想这么算一下，但是用如下的语句会报错
%let total=%SYSFUNC(sum(of &var1.-&var3.));

错误提示如下：
ERROR: A character operand was found in the %EVAL function or %IF condition where a numeric
       operand is required. The condition was: of 30-30
ERROR: Argument 1 to function SUM referenced by the %SYSFUNC or %QSYSFUNC macro function is not
       a number.
ERROR: Invalid arguments detected in %SYSCALL, %SYSFUNC, or %QSYSFUNC argument list.  Execution
       of %SYSCALL statement or %SYSFUNC or %QSYSFUNC function reference is terminated.

有好的解决方法吗？谢谢！