#include <stdio.h>

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