#include<bits/stdc++.h>
#define N 100010
#define min(a,b) ((a)<(b)?(a):(b))
#define max(a,b) ((a)>(b)?(a):(b))
#define fsb(a,b,c) for(int (a)=(b);(a)<=(c);(a)++)
#define fbs(a,b,c) for(int (a)=(b);(a)>=(c);(a)--)
using namespace std;
struct node{
 int ls,rs,l1,l2,r1,r2;
}a[N];
int n,mx=0,mn=99999999,ans=0,flag=1;
template<typename qw>inline void rll(qw &x){
 x=0;qw f=1;char c=getchar();
 while(!isdigit(c))f=c=='-'?-1:f,c=getchar();
 while(isdigit(c))x=x*10+c-'0',c=getchar();
 x*=f;
}
inline void dfs(int u,int d){
 if(u==-1)return;
 if(a[u].ls==-1||a[u].rs==-1)mx=max(mx,d),mn=min(mn,d);
 dfs(a[u].ls,d+1);dfs(a[u].rs,d+1);
}
inline void dfs2(int u,int d){
 int ls=a[u].ls,rs=a[u].rs,&l1=a[u].l1,&l2=a[u].l2,&r1=a[u].r1,&r2=a[u].r2;
 if(ls==-1){
  if(d==mn)l1=1;else l2=1;
 }else{
  dfs2(ls,d+1);
  if(flag==0)return;
  l1=a[ls].l1|a[ls].r1;
  l2=a[ls].l2|a[ls].r2;
 }
 if(rs==-1){
  if(d==mn)r1=1;else r2=1;
 }else{
  dfs2(rs,d+1);
  if(flag==0)return;
  r1=a[rs].l1|a[rs].r1;
  r2=a[rs].l2|a[rs].r2;
 }
 if(l1+l2+r1+r2==4){
  flag=0;return;
 }
 if(l1==1&&r2==1)ans++;
}
int main(){
 rll(n);
 fsb(i,1,n){
  rll(a[i].ls);rll(a[i].rs);
 }
 dfs(1,1);
 if(mx-mn>1)flag=0;else dfs2(1,1);
 printf("%d\n",(flag==0)?-1:ans);
 return 0;
}