unary minus in shunting yard expression parser

here is my expression parser using shunting-yard algorithm it work well as expected except in one situation , when I use unary minus like in -2*3 it wont work (I think it shouldn't because I didn't find anything in algorithm to handle this ) is there a simple way that I can fix this? (this is a simple parser I only need () + - * / ^ ) Regards Pedram

#include <cctype>
#include <iostream>
#include <cstring>
#include <cstdlib>
#include <cmath>
using namespace std;
int olaviat (char c) {
   /*************
    **Operator precedence 
    *************/
  switch(c) {
       case '-' : case '+' :
           return 1 ;
       case '*' : case '/' :
           return 2 ;
       case '^' :
           return 3 ;
       default :
           return 0 ;
  }
}
double eval(char *exp) {
    /*************
    **Convert to reverse polish
    *************/
    char n [50] , o[50] ;
    static int nl = 0  , ol = 0 ;

    while (*exp) {
            while(isspace(*exp)) *exp++ ;
        if(*exp == '(') {
             o[ol++]  = *exp++ ;
           }
        else if (*exp == ')'){
            while(o[--ol]!='('){
                    n[nl++] = o[ol];
                    n[nl++] = ' ';
                  }
                  *exp++;
        }
        else if (isdigit(*exp)) {
          while (isdigit(*exp)) {
            n[nl++] = *exp++ ;
          }
        n[nl++] = ' ' ;
        }
        else if (strchr("+-*/^",*exp)){
            if(olaviat(*exp) > olaviat(o[ol-1])) {
               o[ol++]  = *exp++ ;


            }
            else {
                    if(olaviat(*exp) == olaviat(o[ol-1]) && olaviat(*exp)== 3) {
                      o[ol++]  = *exp++ ;
                    }else{
                n[nl++] = o[ol-1] ;
                n[nl++] = ' ' ;
                o[--ol] = '\0' ;

                    }
            }
        }

    }

for (int k = ol-1 ; k >= 0 ; k --){
    n[nl++] = o[k];
    n[nl++] = ' ' ;
}
/*****