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