#include<cstdio>

#include<cstring>

#include<cstdlib>

inline int read()

{

    int x;char c;

    while((c=getchar())<''||c>'');

    for(x=c-'';(c=getchar())>=''&&c<='';)x=x*+c-'';

    return x;

}

#define MN 8000

#define MV 16000

#define ME 56000

#define INF 0x3FFFFFFF

struct edge{int nx,t,l,w;}e[ME*+];

int S=MV+,T=MV+,h[MV+],en=,d[MV+],q[MV+],qn,c[MV+];

int cnt=,L[MN+],R[MN+],u[MN+],z[MN+],fa[MN+];

inline void ins(int x,int y,int l,int w)

{

    e[++en]=(edge){h[x],y,l,w};h[x]=en;

    e[++en]=(edge){h[y],x,,};h[y]=en;

}

bool build(int k,int l,int r)

{

    u[k]=read();

    if(k>&&u[k]&&!u[fa[k]])exit(*puts("OwO"));

    if(l<r)

    {

        int mid;

        fa[L[k]=++cnt]=k;build(L[k],l,mid=read());

        fa[R[k]=++cnt]=k;build(R[k],mid+,r);

        z[L[k]]=R[k];

        ins(k,L[k],,INF);

    }

    L[k]=l;R[k]=r;

}

bool bfs()

{

    int i,j;

    memset(d,,sizeof(d));

    for(d[q[i=qn=]=S]=;i<=qn;++i)for(j=c[q[i]]=h[q[i]];j;j=e[j].nx)

        if(e[j].w&&!d[e[j].t])d[q[++qn]=e[j].t]=d[q[i]]+;

    return d[T];

}

int dfs(int x,int r)

{

    if(x==T)return r;

    int k,u=;

    for(int&i=c[x];i;i=e[i].nx)if(e[i].w&&d[e[i].t]==d[x]+)

    {

        k=dfs(e[i].t,r-u<e[i].w?r-u:e[i].w);

        u+=k;e[i].w-=k;e[i^].w+=k;

        if(u==r)return u;

    }

    return d[x]=,u;

}

int main()

{

    int n=read(),i,j;

    build(,,n);

    for(i=;i<*n;++i)if(u[i])

    {

        ins(S,i,,INF);

        for(j=i;++j<*n;)if(L[i]<=L[j]&&R[i]>=R[j]&&u[j])u[i]=;

        ins(i,i+*n,u[i],INF);

        ins(i+*n,T,,INF);

        for(j=i;++j<*n;)if(R[i]+==L[j]&&z[i]!=j){ins(i+*n,j,,INF);break;}

    }

    for(i=;i<=T;++i)for(j=h[i];j;j=e[j].nx)d[i]-=e[j].l,d[e[j].t]+=e[j].l,e[j].w-=e[j].l;

    for(S+=,T+=,i=;i<S;++i)

    {

        if(d[i]<)ins(i,T,,-d[i]);

        if(d[i]>)ins(S,i,,d[i]);

    }

    while(bfs())dfs(S,INF);

    ins(T-,S-,,INF);

    while(bfs())dfs(S,INF);

    for(i=h[S-];i;i=e[i].nx)if(e[i].t==T-)printf("%d",e[i].w);

}