#include<stdio.h>
int main ()
{
    int n;
    scanf("%d",&n);
    int a[n];
    for(int i=0;i<n;i++)
    {
        scanf("%d",a[i]);
    }
    for(int i=0;i<n;i++)
    {int count =0;
        if(a[i]==a[i+1])
        {
            count++;
        }
    }
    int count=0,i;
    if(count%2==1)
    {
        printf("%d",a[i]);
    }
    else if(count%2!=1)
    {
        printf("-1");
    }
    else
    {
        printf("Invalid input");
        return 0;
    }
}