#include <stdio.h>

int main(){
   int n,coin;scanf("%d %d",&n,&coin);
   if(coin > n){
      printf("%d",-1);
      return 0;
   }
   int a[100000],b[100000];for(int i = 0;i < n-1;i++)scanf("%d %d",&a[i],&b[i]);
   int ans;
   int count[100000];
   for(int i = 0;i < n;i++){
      for(int j = i;j < n;j++){
         if(a[j] == i+1)count[i]++;
      }
   }
   //for(int i = 0;i < n;i++)
   //   printf("count[%d] = %d",i+1,count[i]);
   
   coin--;
   for(int i = 0;i < n;i++){
      //printf("rp:%d\n",i);
      //printf("%d:coin = %d,count[i] = %d\n",i,coin,count[i]);
      if(coin <= 0)break;
      if(coin >= count[i]){
         ans += count[i];
         //printf("%d:coin - count[i] = %d - %d\n",i,coin,count[i]);
         coin -= count[i];
      }else {
         ans += coin;
      }
   }
   printf("%d",ans);
}