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java：

import java.math.BigInteger;
import java.util.Scanner;

public class Main {
	static int N = 5005;
	static BigInteger[] er = new BigInteger[N];
	static BigInteger E = new BigInteger("2");
	static int[] a = new int[N];
	static int[] mai = new int[N];


	public static void main(String args[]) {
		Scanner cin = new Scanner(System.in);
		int n = cin.nextInt();
		er[0] = BigInteger.valueOf(1);
		for (int i = 1; i < N; i++)	er[i] = er[i - 1].multiply(E);
		for (int i = 0; i < n; i++) {
			String s = cin.next();
			int now = cin.nextInt();
			if (s.equals("sell") == true)now = -now-1;
			a[i] = now;	
			mai[i] = -1;
		}
		for(int i = 0; i < n; i++) {
			int maxx = -1;
			for(int j = n-1;j >=0; j--)
			{
				if(mai[j]==-1 && a[j]<0)
					if(maxx==-1 || a[maxx]>a[j])
						maxx = j;
			}
			if(maxx==-1)break;
			int find = -1;
			for(int j=maxx-1; j>=0; j--) {
				if(mai[j]==-1 && a[j]==(-a[maxx]-1))
				{find = j; break;}
				else if(mai[j]==1)
				{find = -1;break;}
			}
			if(find==-1){mai[maxx]=0;continue;}
			for(int j=find;j<=maxx;j++)mai[j]=0;
			mai[maxx] = 1;
		}
		BigInteger ans = new BigInteger("0");
		for (int i = 0; i < n; i++)
			if (mai[i] == 1) {
				ans = ans.add(er[-a[i]-1]);
			}
		System.out.println(ans);
	}

}

#include<stdio.h>
#include<iostream>
#include<string.h>
#include<set>
#include<vector>
#include<map>
#include<math.h>
#include<queue>
#include<string>
#include<stdlib.h>
#include<algorithm>
using namespace std;
#define N 5005
int er[N][N];
int ans[N];
void mul(int *y,int *x,int hehe) {//x = y*hehe;
	y[0] = 0;
	for(int i = 0; i < N; i++) {
		y[i] += x[i]*hehe;
		y[i+1] = y[i]/10;
		y[i] %= 10;
	}
}
void add(int *ans,int *x){//ans += x;
	for(int i = 0; i < N; i++) {
		ans[i] += x[i];
		if(ans[i]>=10){
			ans[i+1] += ans[i]/10;
			ans[i]%=10;
		}
	}
}
int a[N];
int mai[N];
int main(){
	int i, j, n, now;
	memset(er[0], 0, sizeof er[0]);
	er[0][0] = 1;
	for(i=1;i<N;i++)mul(er[i], er[i-1], 2);	
	while(~scanf("%d",&n)){
		memset(mai, -1, sizeof mai);
		for(i=0;i<n;i++) {
			char s[10]; scanf("%s",s); scanf("%d",&now);
			if(s[0]=='s')now=-now-1;
			a[i] = now;
		}
		for(i=0;i<n;i++) {
			int maxx = -1;
			for(j=n-1;j>=0;j--)
				if(mai[j]==-1 && a[j]<0)
					if(maxx==-1 || a[maxx]>a[j])
						maxx = j;
			if(maxx == -1)break;
			int find = -1;
			for(j=maxx-1; j>=0; j--) {
				if(mai[j]==-1 && a[j]==(-a[maxx]-1))
				{find = j; break;}
				else if(mai[j]==1){find=-1;break;}
			}
			if(find==-1){mai[maxx] = 0;continue;}
			for(j=find;j<=maxx;j++)mai[j]=0;
			mai[maxx] = 1;
		}
		memset(ans, 0, sizeof ans);
		for(i = 0; i < n; i++)
			if(mai[i] == 1)
				add(ans, er[-a[i]-1]);
		int pos = N-1;
		while(ans[pos]==0)pos--;
		if(pos<0)pos=0;
		for(;pos>=0;pos--)printf("%d",ans[pos]);
		puts("");
	}
	return 0;
}