#include<stdio.h>
int main(){
    int n;
    scanf("%d",&n);
    if(n<=-10 && n<=10){
        printf("Invalid input");
    }
    int arr[5];
    scanf("%d",arr[]);
    if(arr[n]<=-10 && arr[n]>=10){
        printf("Invalid input");
        return 0;
    }
    for (int i=0;i<=n,i++);{
        int d=0;
        for (int j=0;i<j,j++){
         if(arr[i] == arr[j]){
            d=1;
         }
    }
    if(!d){
        printf("%d",a[i]);
    }
    else{
        printf("Invalid input")
    }
        
        
    }
    
    
}