#include"bits/stdc++.h"
using namespace std; 
#define pb push_back
#define eb emplace_back
#define ins insert
#define all(x) x.begin(),x.end()
#define rall(x) x.rbegin(),x.rend()
#define rep(i,a,b) for(int i=a;i<=b;i++)
#define repr(i,a,b) for(int i=a;i>=b;i--)
#define lb lower_bound
#define ub upper_bound
#ifndef ONLINE_JUDGE
#define _getchar_nolock getchar_unlocked
#define pc putchar_unlocked
#else 
#define pc _putchar_nolock
#endif
#define F1 Flusher(); pc(' ');
#define F2 Flusher(); pc('\n');
#define sz(x) (int)x.size()
#define pii pair<int,int>
#define fir first
#define sec second
#define popcount __builtin_popcount 
#pragma GCC target ("avx2")
#pragma GCC optimization ("O3")
#pragma GCC optimization ("unroll-loops")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
using ll=long long int;
using ld=long double;
#define mp make_pair
#define mt make_tuple
#define bit(x,j) (((ll)x>>j)&1)
#define um unordered_map 
#define mem(a,val) memset(a,val,sizeof(a))
void __print(int x) {cerr << x;} 
void __print(long x) {cerr << x;}
void __print(long long x) {cerr << x;} 
void __print(unsigned x) {cerr << x;}
void __print(unsigned long x){cerr << x;}
void __print(unsigned long long x) {cerr << x;}
void __print(float x) {cerr << x;} 
void __print(double x) {cerr << x;}
void __print(ld x){cerr << x;} 
void __print(char x) {cerr << '\'' << x << '\'';}
void __print(const char *x){cerr<<'\"'<<x<<'\"';}
void __print(const string &x) {cerr<<'\"'<<x<<'\"';}
void __print(bool x){cerr<<(x?"true":"false");}
template<typename T, typename V>
void __print(const pair<T, V> &x){cerr<<'{'; __print(x.first);cerr<<','; __print(x.second); cerr << '}';}
template<typename T>
void __print(const T &x) {int f = 0; cerr << '{'; for (auto &i: x) cerr << (f++ ? "," : ""), __print(i); cerr << "}";}
void _print() {cerr << "]\n";}
template <typename T, typename... V>
void _print(T t, V... v) {__print(t); if (sizeof...(v)) cerr << ", "; _print(v...);}
template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p) 
{ return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ")";}
template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p) 
{ return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + ")"; } 
void debug_out() { cerr << endl; } 
template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) 
{
  cerr << " " << to_string(H);
  debug_out(T...);
}
#ifndef ONLINE_JUDGE 
#define debug(x...) cerr << "[" << #x << "] = ["; _print(x)
#else
#define debug(x...)
#endif
#ifndef ONLINE_JUDGE 
#define debugt(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define debugt(...) 42
#endif
vector<int>v[100010]; int f[100004]; 
void bfs(int s)
{
  queue<int>q; q.push(s); f[s]=1;  
  while(!q.empty())
  {
   int m=q.front(); q.pop(); 
   for(auto i:v[m])
   {
    if(!f[i]){ f[i]=1; q.push(i); }
   }
  }
}
void solve()
{
 int n,m; cin>>n>>m; int x,y; rep(i,1,m){ cin>>x>>y; v[x].eb(y); v[y].eb(x); }
 int cnt=0; 
 rep(i,1,n)
 {
  if(!f[i]){ bfs(i); cnt++; }
 }
 cout<<cnt-1<<"\n"; 
}
int main()
{
   #ifndef ONLINE_JUDGE
  freopen("input.txt","r",stdin);
  freopen("output.txt","w",stdout);
  #endif
  ios_base::sync_with_stdio(false); cin.tie(0); 
  solve(); 
}

Measured memory_bytes: 9252