#include<stdio.h>
#include<string.h>
#define Max 100
int main()
{
    int q;
    scanf("%d",&q);{
        printf("Invalid input");
        return 0;
    }
    int stack[MAX];
    int top=-1;
    char cmd[10];
    int value;
    for(int i=0;i<q;i++)
    {
        scanf("%s",cmd);
        if(strcmp(cmd,"PUSH")==0)
        {
            scanf("%d",%value);
            if(top<MAX-1)
            {
                stack[++top]=value;
            }
        }
        else if(strcmp(cmd,"POP")==0)
        if(top==-1)
        {
            printf("Stack underflow");
        }
        else
        {
            top--;
        }
    }
    else if(strcmp(cmd,"PEEK")==0)
    {
        if(top==-1)
        {
            printf("Stack underflow\n");
        }
        else
        {
            printf("%d\n",stack[top]);
        }
    }
    else if(strcmp(cmd,"DISPLAY")==0)
    {
        if(top==-1)
        {
            printf("stack underflow");
        }
        else
        {
            for(int j=0;j<=top;j++)
            {
                printf("%d ",stack[j]);
            }
            printf("\n");
        }
    }
}
return 0;
}