text: #include <bits/stdc++.h>
  
  using namespace std;
  
  //Source : https://discuss.codechef.com/questions/82724/way-to-use-big-integer-in-c
  
  const int base = 1000000000;
  const int base_digits = 9;
  
  struct bigint
  {
      vector<int> a;
      int sign;
  
      bigint() : sign(1) { }
  
      bigint(long long v)
      {
          *this = v;
      }
  
      bigint(const string &s)
      {
          read(s);
      }
  
      void operator=(const bigint &v)
      {
          sign = v.sign;
          a = v.a;
      }
  
      void operator=(long long v)
      {
          sign = 1;
          if (v < 0)
              sign = -1, v = -v;
          for (; v > 0; v = v / base)
              a.push_back(v % base);
      }
  
      bigint operator+(const bigint &v) const
      {
          if (sign == v.sign)
          {
              bigint res = v;
  
              for (int i = 0, carry = 0; i < (int) max(a.size(), v.a.size()) || carry; ++i)
              {
                  if (i == (int) res.a.size())
                      res.a.push_back(0);
                  res.a[i] += carry + (i < (int) a.size() ? a[i] : 0);
                  carry = res.a[i] >= base;
                  if (carry)
                      res.a[i] -= base;
              }
              return res;
          }
          return *this - (-v);
      }
  
      bigint operator-(const bigint &v) const
      {
          if (sign == v.sign)
          {
              if (abs() >= v.abs())
              {
                  bigint res = *this;
                  for (int i = 0, carry = 0; i < (int) v.a.size() || carry; ++i)
                  {
                      res.a[i] -= carry + (i < (int) v.a.size() ? v.a[i] : 0);
                      carry = res.a[i] < 0;
                      if (carry)
                          res.a[i] += base;
                  }
                  res.trim();
                  return res;
              }
              return -(v - *this);
          }
          return *this + (-v);
      }
  
      void operator*=(int v)
      {
          if (v < 0)
              sign = -sign, v = -v;
          for (int i = 0, carry = 0; i < (int) a.size() || carry; ++i)
          {
              if (i == (int) a.size())
                  a.push_back(0);
              long long cur = a[i] * (long long) v + carry;
              carry = (int) (cur / base);
              a[i] = (int) (cur % base);
          }
          trim();
      }
  
      bigint operator*(int v) const
      {
          bigint res = *this;
          res *= v;
          return res;
      }
  
      friend pair<bigint, bigint> divmod(const bigint &a1, const bigint &b1)
      {
          int norm = base / (b1.a.back() + 1);
          bigint a = a1.abs() * norm;
          bigint b = b1.abs() * norm;
          bigint q, r;
          q.a.resize(a.a.size());
  
          for (int i = a.a.size() - 1; i >= 0; i--)
          {
              r *= base;
              r += a.a[i];
              int s1 = r.a.size() <= b.a.size() ? 0 : r.a[b.a.size()];
              int s2 = r.a.size() <= b.a.size() - 1 ? 0 : r.a[b.a.size() - 1];
              int d = ((long long) base * s1 + s2) / b.a.back();
              r -= b * d;
              while (r < 0)
                  r += b, --d;
              q.a[i] = d;
          }
  
          q.sign = a1.sign * b1.sign;
          r.sign = a1.sign;
          q.trim();
          r.trim();
          return make_pair(q, r / norm);
      }
  
      bigint operator/(const bigint &v) const
      {
          return divmod(*this, v).first;
      }
  
      bigint operator%(const bigint &v) const
      {
          return divmod(*this, v).second;
      }
  
      void operator/=(int v)
      {
          if (v < 0)
              sign = -sign, v = -v;
          for (int i = (int) a.size() - 1, rem = 0; i >= 0; --i)
          {
              long long cur = a[i] + rem * (long long) base;
              a[i] = (int) (cur / v);
              rem = (int) (cur % v);
          }
          trim();
      }
  
      bigint operator/(int v) const
      {
          bigint res = *this;
          res /= v;
          return res;
      }
  
      int operator%(int v) const
      {
          if (v < 0)
              v = -v;
          int m = 0;
          for (int i = a.size() - 1; i >= 0; --i)
              m = (a[i] + m * (long long) base) % v;
          return m * sign;
      }
  
      void operator+=(const bigint &v)
      {
          *this = *this + v;
      }
  
      void operator-=(const bigint &v)
      {
          *this = *this - v;
      }
  
      void operator*=(const bigint &v)
      {
          *this = *this * v;
      }
  
      void operator/=(const bigint &v)
      {
          *this = *this / v;
      }
  
      bool operator<(const bigint &v) const
      {
          if (sign != v.sign)
              return sign < v.sign;
          if (a.size() != v.a.size())
              return a.size() * sign < v.a.size() * v.sign;
          for (int i = a.size() - 1; i >= 0; i--)
              if (a[i] != v.a[i])
                  return a[i] * sign < v.a[i] * sign;
          return false;
      }
  
      bool operator>(const bigint &v) const
      {
          return v < *this;
      }
  
      bool operator<=(const bigint &v) const
      {
          return !(v < *this);
      }
  
      bool operator>=(const bigint &v) const
      {
          return !(*this < v);
      }
  
      bool operator==(const bigint &v) const
      {
          return !(*this < v) && !(v < *this);
      }
  
      bool operator!=(const bigint &v) const
      {
          return *this < v || v < *this;
      }
  
      void trim()
      {
          while (!a.empty() && !a.back())
              a.pop_back();
          if (a.empty())
              sign = 1;
      }
  
      bool isZero() const
      {
          return a.empty() || (a.size() == 1 && !a[0]);
      }
  
      bigint operator-() const
      {
          bigint res = *this;
          res.sign = -sign;
          return res;
      }
  
      bigint abs() const
      {
          bigint res = *this;
          res.sign *= res.sign;
          return res;
      }
  
      long long longValue() const
      {
          long long res = 0;
          for (int i = a.size() - 1; i >= 0; i--)
              res = res * base + a[i];
          return res * sign;
      }
  
      friend bigint gcd(const bigint &a, const bigint &b)
      {
          return b.isZero() ? a : gcd(b, a % b);
      }
  
      friend bigint lcm(const bigint &a, const bigint &b)
      {
          return a / gcd(a, b) * b;
      }
  
      void read(const string &s)
      {
          sign = 1;
          a.clear();
          int pos = 0;
          while (pos < (int) s.size() && (s[pos] == '-' || s[pos] == '+'))
          {
              if (s[pos] == '-')
                  sign = -sign;
              ++pos;
          }
          for (int i = s.size() - 1; i >= pos; i -= base_digits)
          {
              int x = 0;
              for (int j = max(pos, i - base_digits + 1); j <= i; j++)
                  x = x * 10 + s[j] - '0';
              a.push_back(x);
          }
          trim();
      }
  
      friend istream& operator>>(istream &stream, bigint &v)
      {
          string s;
          stream >> s;
          v.read(s);
          return stream;
      }
  
      friend ostream& operator<<(ostream &stream, const bigint &v)
      {
          if (v.sign == -1)
              stream << '-';
          stream << (v.a.empty() ? 0 : v.a.back());
          for (int i = (int) v.a.size() - 2; i >= 0; --i)
              stream << setw(base_digits) << setfill('0') << v.a[i];
          return stream;
      }
  
      static vector<int> convert_base(const vector<int> &a, int old_digits, int new_digits)
      {
          vector<long long> p(max(old_digits, new_digits) + 1);
          p[0] = 1;
          for (int i = 1; i < (int) p.size(); i++)
              p[i] = p[i - 1] * 10;
          vector<int> res;
          long long cur = 0;
          int cur_digits = 0;
          for (int i = 0; i < (int) a.size(); i++)
          {
              cur += a[i] * p[cur_digits];
              cur_digits += old_digits;
              while (cur_digits >= new_digits)
              {
                  res.push_back(int(cur % p[new_digits]));
                  cur /= p[new_digits];
                  cur_digits -= new_digits;
              }
          }
          res.push_back((int) cur);
          while (!res.empty() && !res.back())
              res.pop_back();
          return res;
      }
  
      typedef vector<long long> vll;
  
      static vll karatsubaMultiply(const vll &a, const vll &b)
      {
          int n = a.size();
          vll res(n + n);
          if (n <= 32)
          {
              for (int i = 0; i < n; i++)
                  for (int j = 0; j < n; j++)
                      res[i + j] += a[i] * b[j];
              return res;
          }
  
          int k = n >> 1;
          vll a1(a.begin(), a.begin() + k);
          vll a2(a.begin() + k, a.end());
          vll b1(b.begin(), b.begin() + k);
          vll b2(b.begin() + k, b.end());
  
          vll a1b1 = karatsubaMultiply(a1, b1);
          vll a2b2 = karatsubaMultiply(a2, b2);
  
          for (int i = 0; i < k; i++)
              a2[i] += a1[i];
          for (int i = 0; i < k; i++)
              b2[i] += b1[i];
  
          vll r = karatsubaMultiply(a2, b2);
          for (int i = 0; i < (int) a1b1.size(); i++)
              r[i] -= a1b1[i];
          for (int i = 0; i < (int) a2b2.size(); i++)
              r[i] -= a2b2[i];
  
          for (int i = 0; i < (int) r.size(); i++)
              res[i + k] += r[i];
          for (int i = 0; i < (int) a1b1.size(); i++)
              res[i] += a1b1[i];
          for (int i = 0; i < (int) a2b2.size(); i++)
              res[i + n] += a2b2[i];
          return res;
      }
  
      bigint operator*(const bigint &v) const
      {
          vector<int> a6 = convert_base(this->a, base_digits, 6);
          vector<int> b6 = convert_base(v.a, base_digits, 6);
          vll a(a6.begin(), a6.end());
          vll b(b6.begin(), b6.end());
          while (a.size() < b.size())
              a.push_back(0);
          while (b.size() < a.size())
              b.push_back(0);
          while (a.size() & (a.size() - 1))
              a.push_back(0), b.push_back(0);
          vll c = karatsubaMultiply(a, b);
          bigint res;
          res.sign = sign * v.sign;
          for (int i = 0, carry = 0; i < (int) c.size(); i++)
          {
              long long cur = c[i] + carry;
              res.a.push_back((int) (cur % 1000000));
              carry = (int) (cur / 1000000);
          }
          res.a = convert_base(res.a, 6, base_digits);
          res.trim();
          return res;
      }
  };
  
  int main()
  {
      ios_base::sync_with_stdio(false),cin.tie(0),cout.tie(0);
      int N;
      bigint Arr[1010],Num[1010];
      bigint Sum = 0;
      cin >> N;
      for (int i = 0; i < N; ++i)
          cin >> Arr[i];
  
      Num[0] = (bigint)1;
      for (int i = 1; i <= 999; ++i)
          Num[i] = (bigint)2 * Num[i - 1] + (bigint)1;
  
      for (int i = 0; i < N; ++i)
      {
          if ( Arr[i] == 1 )
              Sum = Num[i] - Sum;
      }
      cout << Sum << endl;
      return 0;
  }
  