Educational Codeforces Round 66 (Rated for Div. 2)B. Catch Overflow!

You are given a function f written in some basic language. The function accepts an integer value, which is immediately written into some variable x. x is an integer variable and can be assigned values from 0 to 232−1. The function contains three types of commands:

for n — for loop;
end — every command between “for n” and corresponding “end” is executed n times;
add — adds 1 to x.
After the execution of these commands, value of x is returned.

Every “for n” is matched with “end”, thus the function is guaranteed to be valid. “for n” can be immediately followed by “end”.“add” command can be outside of any for loops.

Notice that “add” commands might overflow the value of x! It means that the value of x becomes greater than 232−1 after some “add” command.

Now you run f(0) and wonder if the resulting value of x is correct or some overflow made it incorrect.

If overflow happened then output “OVERFLOW!!!”, otherwise print the resulting value of x.

Input
The first line contains a single integer l (1≤l≤105) — the number of lines in the function.

Each of the next l lines contains a single command of one of three types:

for n (1≤n≤100) — for loop;
end — every command between “for n” and corresponding “end” is executed n times;
add — adds 1 to x.
Output
If overflow happened during execution of f(0), then output “OVERFLOW!!!”, otherwise print the resulting value of x.

Examples
inputCopy
9
add
for 43
end
for 10
for 15
add
end
add
end
outputCopy
161
inputCopy
2
for 62
end
outputCopy
0
inputCopy
11
for 100
for 100
for 100
for 100
for 100
add
end
end
end
end
end
outputCopy
OVERFLOW!!!
Note
In the first example the first “add” is executed 1 time, the second “add” is executed 150 times and the last “add” is executed 10 times. Note that “for n” can be immediately followed by “end” and that “add” can be outside of any for loops.

In the second example there are no commands “add”, thus the returning value is 0.

In the third example “add” command is executed too many times, which causes x to go over 232−1.

#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#define ll long long
#define dd double
using namespace std;

int main() {
	ios::sync_with_stdio(0);
	ll n = 4294967296;
	ll t;
	cin >> t;
	string s;
	ll sum = 0;
	ll res = 1;
	ll x;
	bool f = true;
	stack<ll> st;
	while (t--) {
		cin >> s;
		if (s == "add") {
			sum += res;
			if (sum >= n) {
				sum = n;
				f = false;
			}
		}
		else if (s == "for") {
			cin >> x;
			res *= x;
			if (res > n - 1) {
				res = n;
			}
			st.push(res);
		}
		else {
			st.pop();
			if (st.empty()) {
				res = 1;
			}
			else {
				res = st.top();
			}
		}
	}
	if (!f) {
		cout << "OVERFLOW!!!" << endl;
	}
	else {
		cout << sum << endl;
	}
}