#include<stdio.h>
#include<string.h>
#define max 100
int main(){
    int q;
    scanf("%d",&q);
    if(q<0){
        printf("Invalid input\n");
        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\n");
                }else{
                    for(int j=0;j<=top;j++) {
                        printf("%d",stack[i]);
                    }
                    printf("\n");
                }
            }
        }
    }
    return 0;
}