Problem:
Score : 300 points

Problem Statement

There are N cities numbered 1 through N, and M bidirectional roads numbered 1 through M.
Road i connects City A_i and City B_i.

Snuke can perform the following operation zero or more times:

Choose two distinct cities that are not directly connected by a road, and build a new road between the two cities.

After he finishes the operations, it must be possible to travel from any city to any other cities by following roads (possibly multiple times).

What is the minimum number of roads he must build to achieve the goal?

Constraints

2 \leq N \leq 100,000

1 \leq M \leq 100,000

1 \leq A_i < B_i \leq N

No two roads connect the same pair of cities.

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N M
A_1 B_1
:
A_M B_M

Output

Print the answer.

Sample Input 1

3 1
1 2

Sample Output 1

1

Initially, there are three cities, and there is a road between City 1 and City 2.

Snuke can achieve the goal by building one new road, for example, between City 1 and City 3.
After that,

We can travel between 1 and 2 directly.

We can travel between 1 and 3 directly.

We can travel between 2 and 3 by following both roads (2 - 1 - 3).

Solution in Java (OpenJDK 11.0.6) (judge verdict: Accepted)

import java.io.IOException;
import java.io.InputStream;
import java.util.*;

public class Main {
    public static void main(String[] args) {
        int n = sc.nextInt();
        int m = sc.nextInt();
        UnionFindTree u = new UnionFindTree(n);
        for (int i=0;i<m;i++)u.unite(sc.nextInt()-1,sc.nextInt()-1);
        System.out.println(Math.max(0,u.Number-1));
    }

    public static final long MOD = 1000000007L;
    public static final int tenE9 = 1000000000;
    public static final int INF = Integer.MAX_VALUE/2;
    public static final int dataDigit = 10000;
    static Basic basic = new Basic();
    static FastScanner sc = new FastScanner();
    public static class FastScanner {
        private final InputStream in = System.in;
        private final byte[] buffer = new byte[1024];
        private int ptr = 0;
        private int buflen = 0;
        private boolean hasNextByte() {
            if (ptr < buflen) {
                return true;
            }else{
                ptr = 0;
                try {
                    buflen = in.read(buffer);
                } catch (IOException e) {
                    e.printStackTrace();
                }
                if (buflen <= 0) {
                    return false;
                }
            }
            return true;
        }
        private int readByte() { if (hasNextByte()) return buffer[ptr++]; else return -1;}
        private static boolean isPrintableChar(int c) { return 33 <= c && c <= 126;}
        public boolean hasNext() { while(hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++; return hasNextByte();}
        public String next() {
            if (!hasNext()) throw new NoSuchElementException();
            StringBuilder sb = new StringBuilder();
            int b = readByte();
            while(isPrintableChar(b)) {
                sb.appendCodePoint(b);
                b = readByte();
            }
            return sb.toString();
        }
        public long nextLong() {
            if (!hasNext()) throw new NoSuchElementException();
            long n = 0;
            boolean minus = false;
            int b = readByte();
            if (b == '-') {
                minus = true;
                b = readByte();
            }
            if (b < '0' || '9' < b) {
                throw new NumberFormatException();
            }
            while(true){
                if ('0' <= b && b <= '9') {
                    n *= 10;
                    n += b - '0';
                }else if(b == -1 || !isPrintableChar(b)){
                    return minus ? -n : n;
                }else{
                    throw new NumberFormatException();
                }
                b = readByte();
            }
        }
        public int nextInt() {
            long nl = nextLong();
            if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE) throw new NumberFormatException();
            return (int) nl;
        }
        public double nextDouble() { return Double.parseDouble(next());}
    }
    public static class Basic {
        //math
        private final int MAX = 51*dataDigit;
        private final long MOD = Main.MOD;
        private final long[] fac = new long[MAX];
        private final long[] finv = new long[MAX];
        private final long[] inv = new long[MAX];
        public long factorial(long num) {
            if (num < 2) return 1;
            else return num * factorial(num - 1);
        }//階乗
        public long modFactorial(long num) {
            if (num < 2) return 1;
            else return num*modFactorial(num-1)%MOD;
        }//余り階乗
        public double log(double base, double antilogarithm) {
            return Math.log(antilogarithm) / Math.log(base);
        }//対数
        public long gcd(long x, long y) {
            if (y == 0) return x;
            else return gcd(y, x % y);
        }//最大公約数
        public long lcm(long x, long y) {
            return x / gcd(x, y) * y;
        }//最小公倍数
        public HashMap<Long, Long> factorization(long num) {
            HashMap<Long, Long> hash = new HashMap<>();
            long n = num;
            long count = 2;
            while (n > 1) {
                while (n % count == 0) {
                    n /= count;
                    if (hash.containsKey(count)) hash.put(count, hash.get(count) + 1);
                    else hash.put(count, 1L);
                }
                count++;
            }
            return hash;
        }//素因数分解
        public int sum(int[] num) {
            int ans = 0;
            for (int j : num) {
                ans += j;
            }
            return ans;
        }//総和
        public int[] reduce(int small, int big) {
            int ins = 2;
            while (ins <= small) {
                if (small % ins == 0 && big % ins == 0) {
                    small /= ins;
                    big /= ins;
                    ins = 2;
                } else {
                    ins++;
                }
            }
            return new int[]{small, big};
        }//約分
        public int reduceCount(int small, int big) {
            int ins = 2;
            int ans = 0;
            while (ins <= small) {
                if (small % ins == 0 && big % ins == 0) {
                    small /= ins;
                    big /= ins;
                    ins = 2;
                    ans++;
                } else {
                    ins++;
                }
            }
            return ans;
        }//約分回数
        public long modPow(long a, long n, long mod) {
            long res = 1;
            while (n > 0) {
                if ((n & 1) != 0) res = res * a % mod;
                a = a * a % mod;
                n >>= 1;
            }
            return res;
        }
        public long pow(long a, long n) {
            long res = 1;
            while (n > 0) {
                if ((n & 1) != 0) res = res * a;
                a = a * a;
                n >>= 1;
            }
            return res;
        }
        public long modInv(long a, long mod) {
            return modPow(a, mod - 2, mod);
        }//単位元は素数であることが必須。
        public void dataInit() {
            fac[0] = 1;
            fac[1] = 1;
            finv[0] = 1;
            finv[1] = 1;
            inv[1] = 1;
            for (int i = 2; i < MAX; i++) {
                fac[i] = fac[i - 1] * i % MOD;
                inv[i] = MOD - inv[(int)(MOD % i)] * (MOD / i) % MOD;
                finv[i] = finv[i - 1] * inv[i] % MOD;
            }
        }
        public long combination(int n, int k) {
            if (n < k) return 0;
            if (n < 0 || k < 0) return 0;
            return fac[n] * (finv[k] * finv[n-k]%MOD)% MOD;
        }
        public long permutation(int n, int k){
            return fac[n]*finv[n-k]%MOD;
        }
        public long hCOM(int n, int k) {
            return combination(n + k - 1, k);
        }
        public boolean[] getIsPrimeArray(int max){
            boolean[] ret = new boolean[max+1];
            Arrays.fill(ret,true);
            ret[0]=false;
            ret[1]=false;
            for (int i=2;i<=max;i++){
                if (ret[i]){
                    int c = 2*i;
                    while (c<=max){
                        ret[c]=false;
                        c+=i;
                    }
                }
            }
            return ret;
        }
        public int digits(long n) {
            return String.valueOf(n).length();
        }
        boolean isPrime(int N) {
            if(N <= 1)
                return false;
            else if(N == 2)
                return true;
            for(int i = 2; i <= Math.sqrt(N); i++)
                if(N % i == 0)
                    return false;
            return true;
        }
        public long modInvert(long a, long m){
            long b = m;
            long u = 1;
            long v = 0;
            while (b!=0){
                long t = a/b;
                a-=t*b;
                long c = a;
                a=b;
                b=c;
                u-=t*v;
                c=u;
                u=v;
                v=c;
            }
            u%=m;
            if (u<0)u+=m;
            return u;
        }//単位元が素数でなくても可能。a,mは互いに素であることが必須。得られる逆元は最小値。より高速。



        //array
        public int[] remove(int[] ar, int pos) {
            int[] ret = new int[ar.length - 1];
            for (int i = 0; i < pos; i++) ret[i] = ar[i];
            for (int i = pos + 1; i < ar.length; i++) ret[i - 1] = ar[i];
            return ret;
        }
        public int[] nextArray(int size,int cons){
            int[] ret = new int[size];
            for (int i=0;i<size;i++)ret[i]=sc.nextInt()+cons;
            return  ret;
        }
        public int[] reverse(int[] ar){
            int[] ret = new int[ar.length];
            for (int i=0;i<ar.length;i++){
                ret[i]=ar[ar.length-1-i];
            }
            return ret;
        }
        public long[] cumulative_sum(int[] ar){
            long[] ret = new long[ar.length+1];
            ret[0]=0;
            for (int i=0;i<ar.length;i++)ret[i+1]=ret[i]+ar[i];
            return ret;
        }
        public int median(int[] a) {
            Arrays.sort(a);
            if(a.length % 2 == 1)
                return a[a.length/2];
            else
                return (a[a.length/2-1] + a[a.length/2]) / 2;
        }
        public int binarySearch(int[] array, int target) {
            int pos = -1;
            int left = 0;
            int right = array.length - 1;
            int middle;
            while (pos == -1 && left <= right) {
                middle = (left + right) / 2;
                if (array[middle] == target) pos = middle;
                else if (array[middle] > target) right = middle - 1;
                else left = middle + 1;
            }
            return pos;
        }
        public int lowerBound(int[] a, int key) {
            if(a[a.length-1] < key)
                return a.length;
            int lb = 0, ub = a.length-1, mid;
            do {
                mid = (ub+lb)/2;
                if(a[mid] < key)
                    lb = mid + 1;
                else
                    ub = mid;
            }while(lb < ub);
            return ub;
        }
        public int upperBound(int[] a, int key) {
            if(a[a.length-1] <= key)
                return a.length;
            int lb = 0, ub = a.length-1, mid;
            do {
                mid = (ub+lb)/2;
                if(a[mid] <= key)
                    lb = mid + 1;
                else
                    ub = mid;
            }while(lb < ub);
            return ub;
        }
        public int count(int[] a, int key) {
            return upperBound(a, key) - lowerBound(a, key);
        }
        public List<String> Permutation = new ArrayList<>();
        public void initPermutation(){Permutation.clear();}
        public void permutationList(String q, String ans){
            if(q.length() <= 1) {
                Permutation.add(ans + q);
            }
            else {
                for (int i = 0; i < q.length(); i++) {
                    permutationList(q.substring(0, i) + q.substring(i + 1), ans + q.charAt(i));
                }
            }
        }
        public Comparator<String> dictionarySort = new Comparator<String>() {
            @Override
            public int compare(String o1, String o2) {
                int ret = 0;
                int p = 0;
                int min = Math.min(o1.length(),o2.length());
                boolean b = true;
                while (b&&p<min){
                    if (o1.charAt(p)<o2.charAt(p)){
                        ret=-1;
                        b=false;
                    }else if (o1.charAt(p)>o2.charAt(p)){
                        ret=1;
                        b=false;
                    }
                    p++;
                }
                if (ret==0){
                    if (o1.length()<o2.length())ret=-1;
                    else if (o1.length()>o2.length())ret=1;
                }
                return ret;
            }
        };
        public int[] forEach(int[] array, ArrayReplace ar){
            int[] ret = new int[array.length];
            for (int i=0;i<array.length;i++)ret[i]=ar.replace(i);
            return ret;
        }
        //graph
        public void warshall_floyd(int[][] d) {
            int n = d.length;
            for (int k = 0; k < n; k++)
                for (int i = 0; i < n; i++) for (int j = 0; j < n; j++) d[i][j] = Math.min(d[i][j], d[i][k] + d[k][j]);
        }//隣接行列が引数。辺が無い場合、INFを入力。
        public int[] bellman_ford(int[][] d, int size, int start){
            int e = d.length;
            int[] ans = new int[size];
            Arrays.fill(ans,Integer.MAX_VALUE);
            ans[start]=0;
            for (int i=0;i<size;i++){
                for (int[] a : d) {
                    if (ans[a[1]] > ans[a[0]] + a[2]) {
                        ans[a[1]] = ans[a[0]] + a[2];
                        if (i == size - 1) {
                            System.out.println("negative loop");
                            break;
                        }
                    }
                }
            }
            return ans;
        }//辺を配列の要素として管理。
        public interface ArrayReplace{
            int replace(int num);
        }
    }
    public static class Queue<T>{
        private final int max = 10000000;
        private final T[] queue = (T[]) new Object[max];
        private int tail = 0; private int head = 0;
        public void init(){this.head = 0; tail = 0;}
        public boolean isEmpty(){return (head==tail);}
        public boolean isFull(){return (head == (tail+1)%max);}
        public void enqueue(T v){
            if (isFull()){
                System.out.println("error: queue is full.");
                return;
            }
            queue[tail++] = v;
            if (tail == max) tail = 0;
        }
        public T dequeue(){
            if (isEmpty()){
                System.out.println("error: queue is empty");
                return null;
            }
            T res = queue[head];
            ++head;
            if (head == max) head = 0;
            return res;
        }
    }
    public static class Deque<T>{
        private final int max = 10000000;
        private final T[] queue = (T[]) new Object[max];
        private int tail = 0; private int head = 0;
        public void init(){this.head = 0; tail = 0;}
        public boolean isEmpty(){return (head==tail);}
        public boolean isFull(){
            return (head == (tail+1)%max);
        }
        public void pushFirst(T v){
            if (isFull()){
                System.out.println("error: queue is full.");
                return;
            }
            head--;
            if (head==-1)head=max-1;
            queue[head] = v;
        }
        public void pushLast(T v){
            if (isFull()){
                System.out.println("error: queue is full.");
                return;
            }
            queue[tail++] = v;
            if (tail == max) tail = 0;
        }
        public T popFirst(){
            if (isEmpty()){
                System.out.println("error: queue is empty");
                return null;
            }
            T res = queue[head];
            ++head;
            if (head == max) head = 0;
            return res;
        }
        public T popLast(){
            if (isEmpty()){
                System.out.println("error: queue is empty");
                return null;
            }
            if (--tail==-1)tail=max-1;
            return queue[tail];
        }

        public T peekFirst(){
            if (isEmpty()){
                System.out.println("error: queue is empty");
                return null;
            }
            return queue[head];
        }
        public T peekLast(){
            if (isEmpty()){
                System.out.println("error: queue is empty");
                return null;
            }
            return queue[tail];
        }
    }
    public static class Stack<T>{
        private final int max = 1000000;
        private final T[] stack =(T[]) new Object[max];
        private int top = 0;
        public boolean isEmpty(){return (top==0);}
        public boolean isFull(){return (top==max);}
        public void init(){top=0;}
        public void push(T v){
            if (isFull()){
                System.out.println("error: stack is full.");
                return;
            }
            stack[top++] = v;
        }
        public T pop(){
            if (isEmpty()){
                System.out.println("error: stack is empty");
                return null;
            }
            return stack[--top];
        }
        public T peek(){
            if (isEmpty()){
                System.out.println("error: stack is empty");
                return null;
            }
            return stack[top];
        }
    }
    public static class Pair<K,V>{
        private K A;
        private V B;
        Pair(K a,V b){
            A=a;
            B=b;
        }
        public void changeA(K a){A=a;}
        public void changeB(V b){B=b;}
        public void changeAB(K a,V b){A=a;B=b;}
        public static Comparator numberSortWithA = new Comparator() {
            @Override
            public int compare(Object o1, Object o2) {
                Pair p = (Pair)o1;
                Pair q = (Pair)o2;
                if ((int)p.A<(int)q.A)return -1;
                else if ((int)p.A==(int)q.A){
                    if ((int)p.B<(int)q.B)return -1;
                    else if ((int)p.B==(int)q.B)return 0;
                    else return 1;
                }
                else return 1;
            }
        };
        public static Comparator numberSortWithB = new Comparator() {
            @Override
            public int compare(Object o1, Object o2) {
                Pair p = (Pair)o1;
                Pair q = (Pair)o2;
                if ((int)p.B<(int)q.B)return -1;
                else if ((int)p.B==(int)q.B){
                    if ((int)p.A<(int)q.A)return -1;
                    else if ((int)p.A==(int)q.A)return 0;
                    else return 1;
                }
                else return 1;
            }
        };
    }
    public static class Graph{
        private final HashMap<Integer,HashSet<Integer>> graph = new HashMap<>();
        private final int Size;
        Graph(int size){
            for (int i=0;i<size;i++)graph.put(i,new HashSet<>());
            Size = size;
        }

        public void addEdge(int node1,int node2){
            if (graph.containsKey(node1)&&graph.containsKey(node2)){
                graph.get(node1).add(node2);
                graph.get(node2).add(node1);
            }
        }
        public void addDirectEdge(int from,int to){
            if (graph.containsKey(from)&&graph.containsKey(to)){
                graph.get(from).add(to);
            }
        }
        public int[] depth(int root){
            Queue<Integer> q = new Queue<>();
            q.enqueue(root);
            boolean[] seen = new boolean[Size];
            Arrays.fill(seen, false);
            seen[root]=true;
            int[] depth = new int[Size];
            Arrays.fill(depth,-1);
            depth[root]=0;
            while (!q.isEmpty()){
                int v = q.dequeue();
                for (int u : graph.get(v)){
                    if (!seen[u]){
                        seen[u]=true;
                        q.enqueue(u);
                        depth[u]=depth[v]+1;
                    }
                }
            }
            return depth;
        }
        public HashSet<Integer> getConnected(int pos){return graph.get(pos);}
        public boolean isConnected(int from,int to){return graph.get(from).contains(to);}
        public int diameter(){
            int[] f = depth(0);
            int m = Arrays.stream(f).max().getAsInt();
            int n = basic.binarySearch(f,m);
            int[] s = depth(n);
            return Arrays.stream(s).max().getAsInt();
        }


    }
    public static class WeightedGraph{
        private final HashMap<Integer,HashMap<Integer,Integer>> graph = new HashMap<>();
        private final int Size;
        private long[] Depth;
        WeightedGraph(int size){
            for (int i=0;i<size;i++)graph.put(i,new HashMap<>());
            Size = size;
        }

        public void addEdge(int node1,int node2,int weight){
            if (graph.containsKey(node1)&&graph.containsKey(node2)){
                graph.get(node1).put(node2,weight);
                graph.get(node2).put(node1,weight);
            }
        }
        public void addDirectEdge(int from,int to,int weight){
            if (graph.containsKey(from)&&graph.containsKey(to)){
                graph.get(from).put(to,weight);
            }
        }
        public int[] dijkstra(int root){
            Comparator<int[]> c = new Comparator<int[]>() {
                @Override
                public int compare(int[] o1, int[] o2) {
                    return Integer.compare(o1[1], o2[1]);
                }
            };
            PriorityQueue<int[]> q = new PriorityQueue<>(c);
            q.add(new int[]{root,0});
            int[] ret = new int[Size];
            Arrays.fill(ret,Integer.MAX_VALUE);
            ret[root]=0;
            while (!q.isEmpty()){
                int[] v = q.poll();
                if (v[1]<=ret[v[0]]){
                    HashMap<Integer,Integer> h = graph.get(v[0]);
                    for (int u : h.keySet()){
                        if (ret[u]>v[1]+h.get(u)){
                            ret[u]=v[1]+h.get(u);
                            q.add(new int[]{u,ret[u]});
                        }
                    }
                }
            }
            return ret;
        }

        public long[] Depth(int root){
            Queue<Integer> q = new Queue<>();
            q.enqueue(root);
            boolean[] seen = new boolean[Size];
            Arrays.fill(seen, false);
            seen[root]=true;
            long[] depth = new long[Size];
            depth[root]=0;
            while (!q.isEmpty()){
                int v = q.dequeue();
                for (int u : graph.get(v).keySet()){
                    if (!seen[u]){
                        seen[u]=true;
                        q.enqueue(u);
                        depth[u]=depth[v]+graph.get(v).get(u);
                    }
                }
            }
            Depth = depth.clone();
            return depth;
        }

    }
    public static class UnionFindTree{
        int[] parent;
        int[] rank;
        int[] Size;
        int Number;
        public UnionFindTree(int size){
            this.parent = new int[size];
            this.rank = new int[size];
            this.Size = new int[size];
            makeSet();
        }
        public void makeSet(){
            for (int i=0;i<Size.length;i++){
                parent[i]=i;
                rank[i]=0;
                Size[i]=1;
            }
            Number= Size.length;
        }
        public void unite(int x, int y){
            int xRoot = find(x);
            int yRoot = find(y);
            if (rank[xRoot]>rank[yRoot]){
                parent[yRoot] = xRoot;
                Size[xRoot]+=Size[yRoot];
                Number--;
            }else if (rank[xRoot]<rank[yRoot]){
                parent[xRoot] = yRoot;
                Size[yRoot]+=Size[xRoot];
                Number--;
            }else if (xRoot!=yRoot){
                parent[yRoot] = xRoot;
                rank[xRoot]++;
                Size[xRoot]+=Size[yRoot];
                Number--;
            }
        }
        public int find(int i){
            if (i!= parent[i]){
                parent[i]=find(parent[i]);
            }
            return parent[i];
        }
        public boolean same(int x, int y){
            return find(x) == find(y);
        }
        public int getSize(int i){return Size[find(i)];}
        boolean isConnected(){
            return Number<2;
        }
    }
    public static class BitSearch<T>{
        T[] Array;
        int Width;
        int Length;
        BitSearch(T[] t, int width){
            Array = t;
            Width = width;
            Length = Array.length;
        }
        void start(){
            long max = (long)Math.pow(Width,Length);
            for (long i=0;i<max;i++){
                StringBuilder s = new StringBuilder(Long.toString(i, Width));
                while (s.length()<Length) s.insert(0, "0");
                run(s);
            }
        }
        void run(StringBuilder s){

        }
    }
    public static class SegmentTree{
        static int N, n, INF = Integer.MAX_VALUE/2;
        static int[] dat;
        static void init(int N) {
            n = 1;
            while(n<N)
                n*=2;
            dat=new int[2*n-1];
            for(int i=0;i<2*n-1;i++)
                dat[i]=INF;
        }
        static void update(int k, int a) {
            k+=N-1;
            dat[k]=a;
            while(k>0) {
                k =(k-1)/2;
                dat[k]=Math.min(dat[k*2+1],dat[k*2+2]);
            }
        }
        static int queryMin(int a, int b, int k, int l, int r) {
            if(r<=a||b<=l)
                return INF;
            else if(a<=l&&r<=b)
                return dat[k];
            else
                return Math.min(queryMin(a, b, k*2+1, l, (l+r)/2), queryMin(a, b, k*2+2, (l+r)/2, r));
        }
        static int queryMin(int a, int b) {
            return queryMin(a, b, 0, 0, n);
        }

    }

}