int calculate(const char* expr, char **result)
{
    if(expr == NULL || result == NULL)
        return 1;
    if(validate_expression(expr) != 0)
        return 2;
    *result = malloc(2000);
    if(*result == NULL)
    {
        return 3;
    }
    int i = 0;
    char *ex = (char*)expr;
    int flag = 0;
    if(*ex == '-')
    {
        **result = '-';
        ex++;
        flag = 1;
    }
    if(flag == 1)
        i = 1;
    int k = 0;
    while(*ex != '-' && *ex != '*' && *ex != '+' && *ex != '\0')
    {
        *(*result + i) = *ex;
        i++;
        k++;
        ex++;
    }
    *(*result + i) = '\0';
    if(*ex == '\0')
    {
        return 0;
    }

    char *temp;
    char *res;
    temp = malloc(500);
    if(temp == NULL)
    {
        free(result);
        return 3;
    }
    char znak = *ex;
    k = 0;
    while(*(ex + k) != '\0')
    {
        k++;
    }
    for(i = 1; i < k; i++)
    {
        int j = 0;
        while(*(ex + i) != '+' && *(ex + i) != '*' && *(ex + i) != '\0')
        {

            if(*(ex + i) == '-')
            {
                if(j == 0)
                {
                    *(temp + j) = *(ex + i);
                    i++;
                    j++;
                }
                else
                    break;
            }
            *(temp + j) = *(ex + i);
            j++;
            i++;
        }
        *(temp + j) = '\0';
        if(znak == '+')
        {
            add(*result, temp, &res);
            strcpy(*result, res);
            free(res);
        }
        else if(znak == '-')
        {
            subtract(*result, temp, &res);
            strcpy(*result, res);
            free(res);
        }
        else
        {
            multiply(*result, temp, &res);
            strcpy(*result, res);
            free(res);
        }
        znak = *(ex + i);
    }
    free(temp);
    return 0;