Language: C++

#include <iostream>
#include <stack>
#include <string>
#include <cstdlib>
using namespace std;

int main()
{
	stack<int> st;
	int a, b;
	string s;

	while (cin >> s) {
		if (s[0] == '+') {
			b = st.top(); st.pop();
			a = st.top(); st.pop();
			st.push(a + b);
		}
		else if (s[0] == '-') {
			b = st.top(); st.pop();
			a = st.top(); st.pop();
			st.push(a - b);
		}
		else if (s[0] == '*') {
			b = st.top(); st.pop();
			a = st.top(); st.pop();
			st.push(a * b);
		}
		else {
			st.push(atoi(s.c_str()));
		}
	}
????????????cout << st.top() << endl;
    return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:33:1: error: expected primary-expression before '?' token
   33 | ????????????cout << st.top() << endl;
      | ^
a.cc:33:2: error: expected primary-expression before '?' token
   33 | ????????????cout << st.top() << endl;
      |  ^
a.cc:33:3: error: expected primary-expression before '?' token
   33 | ????????????cout << st.top() << endl;
      |   ^
a.cc:33:4: error: expected primary-expression before '?' token
   33 | ????????????cout << st.top() << endl;
      |    ^
a.cc:33:5: error: expected primary-expression before '?' token
   33 | ????????????cout << st.top() << endl;
      |     ^
a.cc:33:6: error: expected primary-expression before '?' token
   33 | ????????????cout << st.top() << endl;
      |      ^
a.cc:33:7: error: expected primary-expression before '?' token
   33 | ????????????cout << st.top() << endl;
      |       ^
a.cc:33:8: error: expected primary-expression before '?' token
   33 | ????????????cout << st.top() << endl;
      |        ^
a.cc:33:9: error: expected primary-expression before '?' token
   33 | ????????????cout << st.top() << endl;
      |         ^
a.cc:33:10: error: expected primary-expression before '?' token
   33 | ????????????cout << st.top() << endl;
      |          ^
a.cc:33:11: error: expected primary-expression before '?' token
   33 | ????????????cout << st.top() << endl;
      |           ^
a.cc:33:12: error: expected primary-expression before '?' token
   33 | ????????????cout << st.top() << endl;
      |            ^
a.cc:33:37: error: expected ':' before ';' token
   33 | ????????????cout << st.top() << endl;
      |                                     ^
      |                                     :
a.cc:33:37: error: expected primary-expression before ';' token
a.cc:33:37: error: expected ':' before ';' token
   33 | ????????????cout << st.top() << endl;
      |                                     ^
      |                                     :
a.cc:33:37: error: expected primary-expression before ';' token
a.cc:33:37: error: expected ':' before ';' token
   33 | ????????????cout << st.top() << endl;
      |                                     ^
      |                                     :
a.cc:33:37: error: expected primary-expression before ';' token
a.cc:33:37: error: expected ':' before ';' token
   33 | ????????????cout << st.top() << endl;
      |                                     ^
      |                                     :
a.cc:33:37: error: expected primary-expression before ';' token
a.cc:33:37: error: expected ':' before ';' token
   33 | ????????????cout << st.top() << endl;
      |                                     ^
      |                                     :
a.cc:33:37: error: expected primary-expression before ';' token
a.cc:33:37: error: expected ':' before ';' token
   33 | ????????????cout << st.top() << endl;
      |                                     ^
      |                                     :
a.cc:33:37: error: expected primary-expression before ';' token
a.cc:33:37: error: expected ':' before ';' token
   33 | ????????????cout << st.top() << endl;
      |                                     ^
      |                                     :
a.cc:33:37: error: expected primary-expression before ';' token
a.cc:33:37: error: expected ':' before ';' token
   33 | ????????????cout << st.top() << endl;
      |                                     ^
      |                                     :
a.cc:33:37: error: expected primary-expression before ';' token
a.cc:33:37: error: expected ':' before ';' token
   33 | ????????????cout << st.top() << endl;
      |                                     ^
      |                                     :
a.cc:33:37: error: expected primary-expression before ';' token
a.cc:33:37: error: expected ':' before ';' token
   33 | ????????????cout << st.top() << endl;
      |                                     ^
      |                                     :
a.cc:33:37: error: expected primary-expression before ';' token
a.cc:33:37: error: expected ':' before ';' token
   33 | ????????????cout << st.top() << endl;
      |                                     ^
      |                                     :
a.cc:33:37: error: expected primary-expression before ';' token
a.cc:33:37: error: expected ':' before ';' token
   33 | ????????????cout << st.top() << endl;
      |                                     ^
      |                                     :
a.cc:33:37: error: expected primary-expression before ';' token