#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <math.h>
 
int main(int argc, char const *argv[])
{
    int n, A;
    scanf("%d %d",&n,&A);
 
    int *aux = (int*)malloc(sizeof(int)*1000001);
    int i,color;
 
    for(i=0;i<=1000001;i++)
        aux[i] = -1;
 
    int counter=0;
 
    for(i=1;i<=n;i++)
    {
        scanf("%d",&color);
        if(color==A) counter++;
        else {
            if(aux[color]>=counter-1){
                aux[color]++;
            }
            else{
                aux[color] = -2;
            }
        }
    }
    int ok=1;
    for(i=1;i<=1000001;i++){
        if(aux[i]>counter-1){
            printf("%d\n",i);
            ok=0;
            break;
        }
    }
    if(ok==1) printf("-1\n");
    return 0;
}