import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.InputMismatchException;
import java.util.Iterator;
import java.util.List;
import java.util.NoSuchElementException;
import java.util.Objects;
import java.util.stream.IntStream;

public final class Main {
    void solve(MyScanner in, MyWriter out) {
        int n = in.nextInt();
        int m = in.nextInt();
        int[][] ab = in.nextVerticalIntArrays(2, m);
        int[] a = decrement(ab[0]);
        int[] b = decrement(ab[1]);
        var uft = new UnionFindTree(n);
        for (int i = 0; i < m; i++) {
            uft.unite(a[i], b[i]);
        }
        out.println(uft.unionCount() - 1);
    }
    static final class UnionFindTree {
        private final int[] parents;
        private final long[] weights;
        private final int[] size;

        UnionFindTree(int elementCount) {
            if (elementCount < 1)
                throw new IllegalArgumentException();
            parents = new int[elementCount];
            Arrays.setAll(parents, i -> i);

            weights = new long[elementCount];

            size = new int[elementCount];
            Arrays.fill(size, 1);
        }
        int rootOf(int x) {
            if (parents[x] == x)
                return x;
            int root = rootOf(parents[x]);
            weights[x] += weights[parents[x]];
            return parents[x] = root;
        }
        boolean same(int x, int y) {
            return rootOf(x) == rootOf(y);
        }
        boolean unite(int x, int y) {
            return unite(x, y, 0);
        }
        boolean unite(int x, int y, long weight) {
            int xRoot = rootOf(x);
            int yRoot = rootOf(y);
            if (xRoot == yRoot)
                return false;
            long w = weight + weightOf(x) - weightOf(y);
            if (size[xRoot] < size[yRoot]) {
                weights[xRoot] = -w;
                parents[xRoot] = yRoot;
                size[yRoot] += size[xRoot];
            } else {
                weights[yRoot] = w;
                parents[yRoot] = xRoot;
                size[xRoot] += size[yRoot];
            }
            return true;
        }
        long weightOf(int x) {
            rootOf(x);
            return weights[x];
        }
        long diff(int x, int y) {
            if (same(x, y))
                return weightOf(y) - weightOf(x);
            throw new IllegalArgumentException();
        }
        int elementCount() {
            return parents.length;
        }
        int unionCount() {
            return (int)IntStream.range(0, elementCount())
                                 .map(this::rootOf)
                                 .distinct()
                                 .count();
        }
        int sizeOf(int x) {
            return size[rootOf(x)];
        }
    }
    private static int[] decrement(int[] a) {
        for (int i = 0; i < a.length; i++)
            a[i]--;
        return a;
    }
    public static void main(String[] args) {
        MyWriter w = new MyWriter(System.out);
        new Main().solve(new MyScanner(System.in), w);
        w.flush();
    }
    static final class MyScanner {
        private static final int DEFAULT_BUFFER_SIZE = 8192;
        private final InputStream in;
        private final byte[] buffer;
        private int point;
        private int readLength;

        MyScanner(InputStream in) {
            this(in, DEFAULT_BUFFER_SIZE);
        }
        MyScanner(InputStream in, int bufferSize) {
            this.in = Objects.requireNonNull(in);
            buffer = new byte[bufferSize];
        }
        private int readByte() {
            if (point < readLength) {
                int result = buffer[point];
                point += 1;
                return result;
            }
            try {
                readLength = in.read(buffer);
            } catch (IOException e) {
                throw new AssertionError(null, e);
            }
            if (readLength == -1)
                return -1;
            point = 1;
            return buffer[0];
        }
        private static boolean isVisibleChar(int c) {
            return 33 <= c && c <= 126;
        }
        char nextChar() {
            int c = readByte();
            while (!(c == -1 || isVisibleChar(c)))
                c = readByte();
            if (c == -1)
                throw new NoSuchElementException();
            return (char)c;
        }
        String next() {
            return next(16);
        }
        String next(int n) {
            int c = readByte();
            while (!(c == -1 || isVisibleChar(c)))
                c = readByte();
            if (c == -1)
                throw new NoSuchElementException();
            StringBuilder b = new StringBuilder(n);
            do {
                b.append((char)c);
                c = readByte();
            } while (c != -1 && isVisibleChar(c));
            return b.toString();
        }
        long nextLong() {
            int c = readByte();
            while (!(c == -1 || isVisibleChar(c)))
                c = readByte();
            if (c == -1)
                throw new NoSuchElementException();
            boolean minus = false;
            long limit = -Long.MAX_VALUE;
            if (c == '-') {
                minus = true;
                limit = Long.MIN_VALUE;
                c = readByte();
            }
            long n = 0L;
            do {
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                if (n < limit / 10L)
                    throw new InputMismatchException();
                n *= 10L;
                int digit = c - '0';
                if (n < limit + digit)
                    throw new InputMismatchException();
                n -= digit;
                c = readByte();
            } while (c != -1 && isVisibleChar(c));
            return minus ? n : -n;
        }
        int nextInt() {
            long n = nextLong();
            if (n < Integer.MIN_VALUE || n > Integer.MAX_VALUE)
                throw new InputMismatchException();
            return (int)n;
        }
        double nextDouble() {
            return Double.parseDouble(next());
        }
        int[] nextIntArray(int n) {
            int[] result = new int[n];
            for (int i = 0; i < n; i++)
                result[i] = nextInt();
            return result;
        }
        long[] nextLongArray(int n) {
            long[] result = new long[n];
            for (int i = 0; i < n; i++)
                result[i] = nextLong();
            return result;
        }
        char[] nextCharArray(int n) {
            char[] result = new char[n];
            for (int i = 0; i < n; i++)
                result[i] = nextChar();
            return result;
        }
        int[][] next2dIntArray(int h, int w) {
            var result = new int[h][w];
            for (int i = 0; i < h; i++)
                for (int j = 0; j < w; j++)
                    result[i][j] = nextInt();
            return result;
        }
        long[][] next2dLongArray(int h, int w) {
            var result = new long[h][w];
            for (int i = 0; i < h; i++)
                for (int j = 0; j < w; j++)
                    result[i][j] = nextLong();
            return result;
        }
        char[][] next2dCharArray(int h, int w) {
            char[][] result = new char[h][w];
            for (int i = 0; i < h; i++)
                for (int j = 0; j < w; j++)
                    result[i][j] = nextChar();
            return result;
        }
        int[][] nextVerticalIntArrays(int arrayCount, int arrayLength) {
            int[][] result = new int[arrayCount][arrayLength];
            for (int i = 0; i < arrayLength; i++)
                for (int j = 0; j < arrayCount; j++)
                    result[j][i] = nextInt();
            return result;
        }
        long[][] nextVerticalLongArrays(int arrayCount, int arrayLength) {
            long[][] result = new long[arrayCount][arrayLength];
            for (int i = 0; i < arrayLength; i++)
                for (int j = 0; j < arrayCount; j++)
                    result[j][i] = nextLong();
            return result;
        }
        char[][] nextVerticalCharArrays(int arrayCount, int arrayLength) {
            char[][] result = new char[arrayCount][arrayLength];
            for (int i = 0; i < arrayLength; i++)
                for (int j = 0; j < arrayCount; j++)
                    result[j][i] = nextChar();
            return result;
        }
        List<String> nextStringList(int n) {
            var result = new ArrayList<String>(n);
            for (int i = 0; i < n; i++)
                result.add(next());
            return result;
        }
        List<Integer> nextIntList(int n) {
            var result = new ArrayList<Integer>(n);
            for (int i = 0; i < n; i++)
                result.add(nextInt());
            return result;
        }
        List<Long> nextLongList(int n) {
            var result = new ArrayList<Long>(n);
            for (int i = 0; i < n; i++)
                result.add(nextLong());
            return result;
        }
    }
    static final class MyWriter extends PrintWriter {
        MyWriter(OutputStream out) {
            super(out);
        }
        void println(int[] x) {
            println(x, " ");
        }
        void println(int[] x, String delimiter) {
            Objects.requireNonNull(delimiter);
            if (x.length > 0) {
                print(x[0]);
                for (int i = 1; i < x.length; i++) {
                    print(delimiter);
                    print(x[i]);
                }
            }
            println();
        }
        void println(long[] x) {
            println(x, " ");
        }
        void println(long[] x, String delimiter) {
            Objects.requireNonNull(delimiter);
            if (x.length > 0) {
                print(x[0]);
                for (int i = 1; i < x.length; i++) {
                    print(delimiter);
                    print(x[i]);
                }
            }
            println();
        }
        void println(Iterable<?> iterable) {
            println(iterable, " ");
        }
        void println(Iterable<?> iterable, String delimiter) {
            Objects.requireNonNull(delimiter);
            Iterator<?> i = iterable.iterator();
            if (i.hasNext()) {
                print(i.next());
                while (i.hasNext()) {
                    print(delimiter);
                    print(i.next());
                }
            }
            println();
        }
        void printLines(int[] x) {
            println(x, System.lineSeparator());
        }
        void printLines(long[] x) {
            println(x, System.lineSeparator());
        }
        void printLines(Iterable<?> iterable) {
            println(iterable, System.lineSeparator());
        }
    }
}


Measured memory_bytes: 41812