#pragma GCC optimize("Ofast")
#pragma GCC target("avx2")

char*mmap();
#define RD(v) int v=0;{int _c;while(_c=*rp++-48,_c>=0)v=v*10+_c;}
#define RDX(v) int v=0;{int _c;while(_c=*rp++-48,_c>=0)v=v<<4|_c;}

int h[1<<21];

main(){
	char*rp=mmap(0l,1l<<28,1,2,0,0ll);
	RD(n);
	if(n==1){
		write(1,"0",1);
		_exit(0);
	}
	int a0,a1;
	{
		RDX(a);
		h[a]++;
		a0=a;
		a1=a;
		--n;
	}
	while(n--){
		RDX(a);
		if(a!=a1){
			a1=a;
			if(h[a]++&&a!=a0){
				write(1,"-1",2);
				_exit(0);
			}
		}
	}
	if(h[a0]>2||h[a0]>1&&a0!=a1){
		write(1,"-1",2);
	}else if(h[a0]>1){
		write(1,"1",1);
	}else{
		write(1,"0",1);
	}
	_exit(0);
}