Go: package main

import (
    "fmt"
    "bufio"
    "strconv"
    "os"
    "strings"
    "math"
)

var rdr = bufio.NewReaderSize(os.Stdin, 1000000)

func readLine() string {
    buf := make([]byte, 0, 1000000)
    for {
        l, p, e := rdr.ReadLine()
        if e != nil {
            panic(e)
        }
        buf = append(buf, l...)
        if !p {
            break
        }
    }
    return string(buf)
}

func toInt(str string) int  {
    n, _ := strconv.Atoi(str)
    return n
}

func abs(a int) int {
    return int(math.Abs(float64(a)))
}

func getPar(x int, parent[1000]int) int {
    if parent[x] == x {
        return x
    } else {
        parent[x] = getPar(parent[x], parent)
        return parent[x]
    }
}

func main() {
    nm := strings.Split(readLine(), " ")
    n := toInt(nm[0])
    m := toInt(nm[1])
    var parent [1000]int
    for i := 0;i < n; i++ {
        parent[i] = i
    }


    for i := 0; i < m; i++ {
        ab := strings.Split(readLine(), " ")
        pa := getPar(toInt(ab[0]) - 1, parent)
        pb := getPar(toInt(ab[1]) - 1, parent)
        parent[pb] = pa
    }

    cities := make(map[int]int)
    villageCnt := 0
    for i := 0;i < n; i ++ {
        pi := getPar(i, parent)
        if i != pi {
            cities[pi] = 1
        } else {
            villageCnt ++
        }
    }
    cityCnt := len(cities)
    fmt.Println(abs(cityCnt * 2 - villageCnt))
}

C++: #include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i,n) for(int (i)=0;(i)<(int)(n);++(i))
#define all(x) (x).begin(),(x).end()
#define pb push_back
#define fi first
#define se second
#define dbg(x) cout<<#x" = "<<((x))<<endl
template<class T,class U> ostream& operator<<(ostream& o, const pair<T,U> &p){o<<"("<<p.fi<<","<<p.se<<")";return o;}
template<class T> ostream& operator<<(ostream& o, const vector<T> &v){o<<"[";for(T t:v){o<<t<<",";}o<<"]";return o;}

int main(){
    int n,m;
    cin >>n >>m;

    vector<vector<int>> G(n);
    rep(i,m){
        int a,b;
        cin >>a >>b;
        --a;
        --b;
        G[a].pb(b);
        G[b].pb(a);
    }

    int ans = 0;
    vector<bool> vis(n);
    rep(i,n)if(!vis[i]){
        int cc = 0;
        queue<int> que;
        que.push(i);
        vis[i] = true;
        ++cc;
        while(!que.empty()){
            int v = que.front();
            que.pop();
            for(int e:G[v]){
                if(!vis[e]){
                    que.push(e);
                    vis[e] = true;
                    ++cc;
                }
            }
        }

        if(cc==1) ++ans;
        else --ans;
    }
    cout << abs(ans) << endl;
    return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
int group[1010];
int parent(int i){
	if(i==group[i])	return i;
	else return group[i]=parent(group[i]);
}
void merge(int i,int j){
	i=parent(i);	j=parent(j);
	group[i]=group[j];
}

int main(){
	int n;	cin>>n;
	int m;	cin>>m;
	for(int i=1;i<=n;i++)	group[i]=i;
	for(int j=0;j<m;j++){
		int a,b;	cin>>a>>b;
		merge(a,b);
	}
	map<int,int> kind;
	for(int i=1;i<=n;i++)	kind[parent(i)]++;
	int village=0,city=0;
	for(auto it:kind){
		if(it.second==1)	village++;
		else 	city++;
	}
	cout<<abs(village-city)<<endl;
	return 0;
}

C++: #include <cstdlib>
#include <cmath>
#include <climits>
#include <cfloat>
#include <map>
#include <set>
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <sstream>
#include <complex>
#include <stack>
#include <queue>
#include <cstdio>
#include <cstring>
#include <iterator>
#include <bitset>
#include <unordered_set>
#include <unordered_map>
#include <fstream>
#include <iomanip>
#include <cassert>
//#include <utility>
//#include <memory>
//#include <functional>
//#include <deque>
//#include <cctype>
//#include <ctime>
//#include <numeric>
//#include <list>
//#include <iomanip>

//#if __cplusplus >= 201103L
//#include <array>
//#include <tuple>
//#include <initializer_list>
//#include <forward_list>
//
//#define cauto const auto&
//#else

//#endif

using namespace std;


typedef long long ll;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;

typedef vector<int> vint;
typedef vector<vector<int> > vvint;
typedef vector<long long> vll, vLL;
typedef vector<vector<long long> > vvll, vvLL;

#define VV(T) vector<vector< T > >

template <class T>
void initvv(vector<vector<T> > &v, int a, int b, const T &t = T()){
    v.assign(a, vector<T>(b, t));
}

template <class F, class T>
void convert(const F &f, T &t){
    stringstream ss;
    ss << f;
    ss >> t;
}

#undef _P
#define _P(...) (void)printf(__VA_ARGS__)
#define reep(i,a,b) for(int i=(a);i<(b);++i)
#define rep(i,n) reep((i),0,(n))
#define ALL(v) (v).begin(),(v).end()
#define PB push_back
#define F first
#define S second
#define mkp make_pair
#define RALL(v) (v).rbegin(),(v).rend()
#define DEBUG
#ifdef DEBUG
#define dump(x)  cout << #x << " = " << (x) << endl;
#define debug(x) cout << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl;
#else
#define dump(x) 
#define debug(x) 
#endif

#define MOD 1000000007LL
#define EPS 1e-8
#define INF 0x3f3f3f3f
#define INFL 0x3f3f3f3f3f3f3f3fLL
#define maxs(x,y) x=max(x,y)
#define mins(x,y) x=min(x,y)

class UF {
	public:
	static const int ufmax=100052;
	int ufpar[ufmax],ufrank[ufmax],ufcnt[ufmax];
	UF() { init();}
	void init(){int i; rep(i,ufmax) { ufpar[i]=i; ufrank[i]=0; ufcnt[i]=1; } }
	int find(int x) {	return (ufpar[x]==x)?(x):(ufpar[x] = find(ufpar[x]));}
	int operator[](int x) {return find(x);}
	int count(int x) {return ufcnt[find(x)];}
	void unite(int x,int y) {
		x = find(x); y = find(y);
		if(x==y) return;
		if(ufrank[x]<ufrank[y]) ufpar[x]=y, ufcnt[y]+=ufcnt[x];
		else {ufpar[y]=x; ufcnt[x]+=ufcnt[y]; if(ufrank[x]==ufrank[y]) ufrank[x]++;}
	}
};



void mainmain(){
	int n,m;
	cin>>n>>m;
	UF uf;
	rep(i,m){
		int a,b;
		cin>>a>>b;
		a--;
		b--;
		uf.unite(a,b);
	}
	set<int> se;
	int a,b=0;
	rep(i,n){
		if(i==uf[i]) b++;
		else se.insert(uf[i]);
	}
	a=se.size();
	b-=a;
	cout<<abs(a-b)<<endl;
	// cout<<a<<" "<<b<<endl;
	// cout<<abs((int)se.size()-(n-(int)se.size()))<<endl;
}


signed main() {
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout<<fixed<<setprecision(20);
    mainmain();
}

C++: #include <iostream>
#include <vector>

class UnionFind {
  public:
    std::vector<int> par;
    std::vector<int> rank;
    std::vector<int> size;

    UnionFind(int N) : par(N), rank(N), size(N) {        
        for (int i = 0; i < N; i++) {
            par[i]  = i;
            rank[i] = 0;
            size[i] = 1;
        }
    }

    int find(int x)
    {
        if (par[x] == x) {
            return x;
        }
        return par[x] = find(par[x]);
    }

    void unite(int x, int y)
    {
        x = find(x);
        y = find(y);

        if (x == y) { return; }

        if (rank[x] < rank[y]) {
            par[x] = y;
            size[y] += size[x];
        } else {
            par[y] = x;
            size[x] += size[y];
            if (rank[x] == rank[y]) {
                rank[x]++;
            }
        }       
    }

    bool same(int x, int y)
    {
        return find(x) == find(y);
    }
};

int main()
{
    int N, M;
    std::cin >> N >> M;

    UnionFind uf(N);
    for (int i = 0; i < M; i++) {
        int a, b;
        std::cin >> a >> b;
        uf.unite(a - 1, b - 1);
    }
    
    int city = 0;
    int town = 0;
    std::vector<bool> used(N, false);
    
    for (int i = 0; i < N; i++) {
        int x = uf.find(i);
        if (used[x]) { continue; }

        used[x] = true;
        if (uf.size[x] == 1) {
            town++;
        } else {
            city++;
        }
    }

    int diff = town - city;
    std::cout << (diff < 0 ? -diff : diff) << std::endl;
    
    return 0;
}