#include<iostream>
#include<cstdio>
#include<algorithm>
#define mod 1000000007
#define MAXN 1000
#define int long long
using namespace std;
inline int read()
{int x = 0 , f = 1; char ch = getchar();while(ch < '0' || ch > '9'){ if(ch == '-') f = -1;  ch = getchar();}while(ch >= '0' && ch <= '9'){x = x * 10 + ch - '0';ch = getchar();}return x * f;
}int tot=-1,n,L[2*MAXN+5];
int l2[MAXN*2+5],l[MAXN+5],r[MAXN+5];
int f[MAXN+5][MAXN*2+5];
int inv[MAXN+5],p[MAXN+5];main()
{p[0]=inv[0]=p[1]=inv[1]=1;for(int i=2;i<=MAXN;i++){p[i]=1LL*p[i-1]*i%mod;inv[i]=1LL*(mod-mod/i)*inv[mod%i]%mod;}//for(int i=2;i<=MAXN;i++) inv[i]=1LL*inv[i]*inv[i-1]%mod;n=read();for(int i=1;i<=n;i++){l[i]=l2[i*2-1]=read();r[i]=l2[i<<1]=read();l2[i<<1]+=1;}sort(l2+1,l2+n*2+1);for(int j=1;j<=n*2;j++)if(l2[j]!=l2[j-1])l2[++tot]=l2[j];  tot++;for(int i=1;i<tot;i++) L[i]=l2[i]-l2[i-1]; for(int i=1;i<=n;i++){   l[i]=upper_bound(l2,l2+tot,l[i])-l2;r[i]=upper_bound(l2,l2+tot,r[i])-l2;//  cout<<l[i]<<" "<<r[i]<<endl;
    }for(int i=0;i<tot;i++) f[0][i]=1;for(int i=1;i<=n;i++){f[i][0]=1;for(int j=l[i];j<=r[i];j++){f[i][j]=(long long)L[j]*f[i-1][j-1]%mod;//cout<<f[i][j]<<endl;int now=1;long long c=L[j]-1;for(int k=i-1;k;--k)if(l[k]<=j&&j<=r[k]){now++;c=c*(long long)(L[j]+now-2)%mod*inv[now]%mod;if(!c)break;f[i][j]=(f[i][j]+(long long)f[k-1][j-1]*c)%mod;}}for(int j=1;j<tot;j++)f[i][j]=((long long)f[i][j]+f[i-1][j]+f[i][j-1]-f[i-1][j-1]+mod)%mod;//  for(int j=1;j<tot;j++)//  cout<<i<<" "<<j<<" "<<f[i][j]<<endl;
    }cout<<(long long)(f[n][tot-1]-1+mod)%mod;return 0;
}

#include<iostream>
#include<cstdio>
#define MN 600000
#define ll long long
using namespace std;
inline int read()
{int x = 0 , f = 1; char ch = getchar();while(ch < '0' || ch > '9'){ if(ch == '-') f = -1;  ch = getchar();}while(ch >= '0' && ch <= '9'){x = x * 10 + ch - '0';ch = getchar();}return x * f;
}int n,m,fa[MN+5],in[MN+5];
ll w[MN+5],ans=0;struct Heap{Heap *l,*r;ll p;int d;Heap(ll _p):p(_p),l(0),r(0),d(1){};inline friend int dis(Heap*x){return x?x->d:0;}friend Heap* Merge(Heap*x,Heap*y){if(!x) return y;if(!y) return x;if(x->p<y->p) swap(x,y);x->r=Merge(x->r,y);if(dis(x->r)>dis(x->l)) swap(x->l,x->r);x->d=dis(x->r)+1;return x;}
}*s[MN+5],*a,*b;int main()
{n=read();m=read();n+=m;for(int i=2;i<=n;i++){++in[fa[i]=read()];ans+=(w[i]=read());}for(int i=n;i>1;i--){if(!s[i]) s[i]=Merge(new Heap(0),new Heap(0));for(int j=1;j<in[i];j++)s[i]=Merge(s[i]->l,s[i]->r);a=new Heap(s[i]->p+w[i]);s[i]=Merge(s[i]->l,s[i]->r);b=new Heap(s[i]->p+w[i]);s[i]=Merge(s[i]->l,s[i]->r);s[fa[i]]=Merge(s[fa[i]],Merge(s[i],Merge(a,b)));}int top=0;while(s[1])w[++top]=s[1]->p,s[1]=Merge(s[1]->l,s[1]->r);w[top+1]=0;for(int i=top;m;m--,i--) ans-=(w[i]-w[i+1])*m;printf("%lld\n",ans);return 0;
}

#include "gap.h"
#include<algorithm>
#include<iostream>
using namespace std;
#define ll long long
#define INF 1000000000000000000LL
ll s[200005];
ll ans=0;
int cnt=0;ll solve(int x)
{ll l,r;MinMax(0,INF,s+1,s+x);cnt=2;int i=2,j=x-1;for(l=s[1]+1,r=s[x]-1;i<=j;l=s[i++]+1,r=s[j--]-1){MinMax(l,r,s+i,s+j);}for(int i=2;i<=x;i++)ans=max(ans,s[i]-s[i-1]);return ans;
}ll findGap(int T, int N)
{if(T==1) return solve(N);if(N==1) return 0;MinMax(0,INF,s+1,s+2);cnt=2;ll p=(s[2]-s[1]-1)/(N-1)+1;for(ll i=s[1]+1;i<=s[2]-1;i+=p){MinMax(i,min(i+p-1,s[2]-1),s+cnt+1,s+cnt+2);if(s[cnt+1]>0)cnt+=2;}sort(s+1,s+cnt+1);for(int i=2;i<=cnt;i++)ans=max(ans,s[i]-s[i-1]);return ans;
}