#include <algorithm>
using namespace std;
#include <iostream>
#include <string>
#include <vector>
#include <sstream>
#include <cctype>
#include <stack>

int solution(string &S) {
    stringstream ss(S);

    vector<string> commands;
    string command;

    while (getline(ss,command,' ')) {
		commands.push_back(command);
	}

	stack<int> st;

    for (auto i : commands) {
        if (isdigit(i[0])) {
            st.push(stoi(i));
        } else if (isalpha(i[0])) {
            if (i[0] == 'P') {
                st.pop();
            } else if (i[0]=='D') {
                st.push(st.top());
            }
        } else {
            if (st.size() < 2) {
                    cout << i;
                    return -1;
            } else {
                int first = st.top();
                st.pop();
                int second = st.top();
                st.pop();

                if (i[0] == '+') {
                    st.push(first+second);
                } else if (i[0] == '-') {
                    int diff = second - first;
                    if (diff < 0) {
                        cout << i;
                        return -1;
                    } else {
                        st.push(diff);
                    }
                }
            }
        }
    }

    return st.top();
}