#include <bits/stdc++.h>
using namespace std;
inline char gc() {
  static char buf[100000], *p1 = buf, *p2 = buf;
  return p1 == p2 && (p2 = (p1 = buf) + fread(buf, 1, 100000, stdin), p1 == p2)
             ? EOF
             : *p1++;
}
inline long long read() {
  long long x = 0;
  char ch = getchar();
  bool positive = 1;
  for (; !isdigit(ch); ch = getchar())
    if (ch == '-') positive = 0;
  for (; isdigit(ch); ch = getchar()) x = x * 10 + ch - '0';
  return positive ? x : -x;
}
inline void write(long long a) {
  if (a < 0) {
    a = -a;
    putchar('-');
  }
  if (a >= 10) write(a / 10);
  putchar('0' + a % 10);
}
inline void writeln(long long a) {
  write(a);
  puts("");
}
inline void wri(long long a) {
  write(a);
  putchar(' ');
}
inline unsigned long long rnd() {
  return ((unsigned long long)rand() << 30 ^ rand()) << 4 | rand() % 4;
}
const int N = 100005;
vector<int> v[N];
int ans, sz[N];
void GG() {
  puts("-1");
  exit(0);
}
void dfs(int p, int fa) {
  for (auto i : v[p])
    if (i != fa) {
      dfs(i, p);
      sz[p] += sz[i];
    }
  if (abs(sz[p] - 1) > 1) GG();
  sz[p] = -(sz[p] - 1);
  ans += abs(sz[p]);
}
int main() {
  int n = read();
  read();
  for (int i = (int)(1); i <= (int)(n - 1); i++) {
    int s = read(), t = read();
    v[s].push_back(t);
    v[t].push_back(s);
  }
  dfs(1, 0);
  if (sz[1]) GG();
  cout << ans << endl;
}
