#include<bits/stdc++.h>
using namespace std;

#define int             long long
#define ld              long double
#define F               first
#define S               second
#define PB              push_back
#define forn(i,a,b)     for(i=a;i<=b;i++)
#define nfor(i,a,b)     for(i=a;i>=b;i--)
#define all(x)          x.begin(),x.end()
#define rall(x)         x.rbegin(),x.rend()
#define clr(x,f)        memset(x,f,sizeof(x))
#define pii             pair<int,int>
#define vi              vector<int>
#define vpii            vector<pii>
#define vvi             vector<vi>
#define si              set<int>
#define spii            set<pii>
#define usi             unordered_set<int>
#define uspii           unordered_set<pii>
#define mii             map<int,int>
#define umii            unordered_map<int,int>
#define pqmx            priority_queue<int>
#define pqmn            priority_queue<int,vi,greater<int>>
#define setbits(x)      __builtin_popcountll(x)
#define tzbits(x)       __builtin_ctzll(x)
#define lzbits(x)       __builtin_clzll(x)
#define pbits(x)        __builtin_parityll(x)
#define gcd             __gcd
#define lcm(x, y)       ((x)*(y))/gcd(x,y)
#define endl            '\n'
#define sz(s)           (int)s.size()
#define sp(x,y)         fixed<<setprecision(y)<<x
const int mod = 1000000007;
const int inf = 1000000000000000000;
const ld PI = 3.1415926535897932384626;
const ld eps = 1e-12;
void __print(int32_t x) {cout<<x;}
void __print(int x) {cout<<x;}
void __print(double x) {cout<<x;}
void __print(ld x) {cout<<x;}
void __print(float x) {cout<<x;}
void __print(bool x) {cout<<(x?"true":"false");}
void __print(char x) {cout <<'\''<<x<<'\'';}
void __print(const char *x) {cout <<'\"' <<x<<'\"';}
void __print(const string &x) {cout<<'\"'<<x<<'\"';}
template<typename T,typename V>     void __print(const pair<T,V> &x) {cout<<'{';__print(x.first);cout<<',';__print(x.second);cout<<'}';}
template<typename T>                void __print(const T &x) {int f=0;cout<<'{';for(auto &i:x)cout<<(f++?",":""),__print(i);cout<<"}";}void _print(){cout<<"]\n";}
template <typename T,typename... V> void _print(T t, V... v) {__print(t);if(sizeof...(v))cout<<", ";_print(v...);}
#define dbg(x...) cout<<"["<<#x<<"]=[";_print(x)
int powerm(int base,int exp) {int res=1;base%=mod;while(exp>0){if(exp&1)res=(res*base)%mod;base=(base*base)%mod;exp=exp>>1;}return res;}
int power(int base,int exp) {int res=1;while(exp>0){if(exp&1)res=res*base;base=base*base;exp=exp>>1;}return res;}
float powerNeg(float base,int exp) {float temp;if(exp==0)return 1;temp=powerNeg(base,exp/2);if(exp%2==0)return temp*temp;else{if(exp>0)return base*temp*temp;else return (temp*temp)/base;}}
int modinv(int exp) {return powerm(exp,mod-2);}
// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++


const int N=2e6+5;

vi p,s;
int comp;
int get(int a){
    return (p[a]==a?a:get(p[a]));
}
void merge(int a,int b){
    a=get(a);
    b=get(b);
    if(a!=b){
        if(s[a]<s[b]) swap(a,b);
        p[b]=a;
        s[a]+=s[b];
        comp--;
    }
}
void solve(){
    
    int i,j;
    int n,m; cin>>n>>m;

    p=vi(n+1), s=vi(n+1);
    forn(i,1,n) p[i]=i,s[i]=1;
    comp=n;
    
    forn(i,0,m-1){
        int a,b; cin>>a>>b;
        merge(a,b);
    }
    cout<<(comp-1)<<endl;
}


int32_t main(){

    ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    
    int t,T=1;
    // cin>>T;
    forn(t,1,T){
        // cout<<"Case #"<<i<<": ";
        solve();
    }
    // cout<<endl<<"Times Elapsed:"<<1.0*clock()/CLOCKS_PER_SEC<<"sec";
    return 0;
}

Measured memory_bytes: 4784