code

#include <bits/stdc++.h>
using namespace std;

bool mlest;

double tlest, tleed;

int R(){
	int x = 0, f = 1; char ch = getchar();
	while(!isdigit(ch)) { if(ch == '-') f = -1; ch = getchar(); }
	while(isdigit(ch)) { x = (x << 1) + (x << 3) + (ch ^ 48); ch = getchar(); }
	return x * f;
}

void W(int x){
	if(x < 0) { x = -x; putchar('-'); }
	if(x > 9) W(x/10); putchar(x%10+'0');
}

const int N = 1e5 + 10;

int n, a[N];

int st[N], top;

int r[N], tmp[N], l[N];

bool k;

void read(){
	n = R();
	for(int i = 1;i <= n; i++) {
		tmp[i] = a[i] = R();
		if(a[i] == 0) k = 1;
	}
	sort(tmp+1,tmp+1+n);
	int m = unique(tmp + 1, tmp + 1 + n) - tmp - 1;
	for(int i = 1;i <= n; i++) a[i] = lower_bound(tmp + 1, tmp + 1 + m, a[i]) - tmp;
}

void init(){
	for(int i = 1;i <= n; i++){
		if(!l[a[i]]) l[a[i]] = i;
		r[a[i]] = i;
	}
}

void compute(){
	int ans = 0;
	for(int i = 1;i <= n; i++){
		if(a[i] == 1 && k){
			if(top){
				W(-1);
				return ;
			}
			continue;
		}
		if(i == l[a[i]]){
			st[top++] = a[i];
			ans = max(ans,top);
		}
		if(i == r[a[i]]){
			if(st[top-1] != a[i]) {
				W(-1);
				return ;
			}
			top--;
		}
		if (i!=l[a[i]] && i!=r[a[i]]) {
			if(st[top-1] != a[i]){
				W(-1);
				return ;
			}
		}
	}
	W(ans);
}

void clear(){
	
}

void run() { read(); init(); compute(); clear(); }

bool mleed;

void wa() { cout << '\n' << tleed-tlest << "ms\n" << (&mleed-&mlest-1)/1024.0/1024.0 << "MB\n"; }

void fre(string s){
	freopen((s+".in").c_str(),"r",stdin);
	freopen((s+".out").c_str(),"w",stdout);
}

int main(){
//	fre("");
	tlest = clock();
	run();
	tleed = clock();
//	wa();
	return 0;
}