#include <bits/stdc++.h>
using namespace std;
namespace IO {
	#if ONLINE_JUDGE
	#define getc() (IS == IT && (IT = (IS = ibuf) + fread(ibuf, 1, IL, stdin), IS == IT) ? EOF : *IS++)
	#else
	#define getc() getchar()
	#endif
	const int IL = 1 << 21, OL = 1 << 21;
	int olen = 0;
	char ibuf[IL], *IS = ibuf, *IT = ibuf, obuf[OL];
	inline int read() {
		register char ch = getc(); register int x = 0, f = 1;
		while(!isdigit(ch)) { if(ch == '-') f = -1; ch = getc(); }
		while(isdigit(ch)) x = x * 10 + ch - 48, ch = getc();
		return x * f;
	}
	inline double readdb() {
		register char ch = getc(); register double x = 0, f = 1;
		while(!isdigit(ch)) { if(ch == '-') f = -1; ch = getc(); }
		while(isdigit(ch)) x = x * 10 + ch - 48, ch = getc();
		if(ch == '.') {
			register double b = 0.1;
			ch = getc();
			while(isdigit(ch)) x += (ch - 48) * b, b *= 0.1, ch = getc();
		}
		return x * f;
	}
	inline int readstr(char *s) {
		register char ch = getc(); register int len = 0;
		while(!isalpha(ch)) ch = getc();
		while(isalpha(ch)) s[++len] = ch, ch = getc();
		return len;
	}
	inline void flush() { fwrite(obuf, 1, olen, stdout); olen = 0; }
	inline void putc(register char ch) { obuf[olen++] = ch; }
	template<class T>
	inline void write(register T x) {
		if(x < 0) obuf[olen++] = '-', x = -x;
		if(x > 9) write(x / 10);
		obuf[olen++] = x % 10 + 48;
	}
} using namespace IO;
const int N = 3e5 + 10;
int n, a[N], cnt[N];
set<int> s1, s2;
bool vis[N];
int main() {
	n = read();
	for(int i = 1; i <= n; i++)
		cnt[i] = 2;
	for(int i = 1; i <= n; i++)
		a[i] = read(), cnt[a[i]]--;
	if(n == 1) {
		puts("-1");
		return 0;
	}
	bool flg = 0;
	for(int i = 1; i <= n; i++) {
		if(cnt[i] != 1) flg = 1;
		if(cnt[i] == 1) s1.insert(i);
		if(cnt[i] == 2) s2.insert(i);
	}
	if(!flg) {
		printf("%d\n", n + 2);
		printf("%d ", a[n]);
		for(int i = 1; i <= n - 1; i++)
			printf("%d ", a[i]);
		printf("%d ", a[1]);
		printf("%d ", a[1]);
		return 0;
	}
	int x = *s2.begin();
	printf("%d\n", n);
	printf("%d ", x);
	int top = 1;
	for(int i = 1; i < n; i++) {
		vis[a[i]] = 1;
		if(!cnt[a[i + 1]] && vis[a[i + 1]]) {
			cnt[a[i + 1]] = 2;
			printf("%d ", x);
			x = a[i + 1];
			if(i + 1 != n)printf("%d ", *s2.begin());
			s2.erase(s2.begin());
			i++;
			continue;
		}
		if(cnt[a[i]] == 1) {
			printf("%d ", a[i]);
			s2.insert(a[i]);
		}
		else {
			printf("%d ", *s2.begin());
			s2.erase(s2.begin());
			continue;
		}
	}
	return 0;
}