#include <stdio.h>
int main() {
    int n;
    scanf("%d",&n);
    if(n<1){
        printf("Invalid input");
    }
    int arr[n],freq[100]={0};
    for(int i=0;i<n;i++){
        scanf("%d ",&arr[i]);
        if(arr[i]<0){
            printf("Invaild input");
        }
        freq[arr[i]]++;
    }
    int found=0;
    for(int i=0;i<100;i++){
        if(freq[i]%2==1){
            printf(" %d",i);
            found=1;
        }
    }
    if(found==2){
        printf("-1")
    }
return 0;
}