// package atcoder.agc.agc004;

import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.InputMismatchException;

public class Main {
    public static void main(String[] args) {
        InputReader in = new InputReader(System.in);
        PrintWriter out = new PrintWriter(System.out);

        int n = in.nextInt();
        int m = in.nextInt();
        if (n == m) {
            throw new RuntimeException("nya-n");
        }
        int[][] graph = buildGraph(in, n, m);

        out.println(solve(graph));
        out.flush();
    }

    private static long solve(int[][] graph) {
        int n = graph.length;
        int[][] pco = parentCountOrder(graph, 0);
        int[] par = pco[0];
        int[] cnt = pco[1];
        int[] ord = pco[2];
        int[] col = new int[n];
        int oneCount = 0;
        for (int oi = 0 ; oi < n ; oi++) {
            int i = ord[oi];
            for (int to : graph[i]) {
                if (to == par[i]) {
                    continue;
                }
                col[to] = col[i]^1;
            }
            oneCount += col[i];
        }
        if (oneCount * 2 != n) {
            return -1;
        }

        long req = 0;
        int[] ones = new int[n];
        for (int oi = n-1 ; oi >= 0 ; oi--) {
            int i = ord[oi];
            int p = par[i];
            ones[i] += col[i];
            if (p != -1) {
                ones[p] += ones[i];
                // proc
                req += Math.max(cnt[i]-ones[i], ones[i]) - Math.min(cnt[i]-ones[i], ones[i]);
            }
        }
        return req;
    }

    public static int[][] parentCountOrder(int[][] graph, int root) {
        int n = graph.length;
        int[] que = new int[2*n];
        int[] parent = new int[n];
        int[] bfsOrd = new int[n];
        int[] cnt = new int[n];
        int qh = 0, qt = 0;
        que[qh++] = root;
        que[qh++] = -1;
        int vi = 0;
        while (qt < qh) {
            int now = que[qt++];
            int par = que[qt++];
            parent[now] = par;
            bfsOrd[vi++] = now;
            for (int to : graph[now]) {
                if (to == par) {
                    continue;
                }
                que[qh++] = to;
                que[qh++] = now;
            }
        }
        for (int i = n-1 ; i >= 0 ; i--) {
            int now = bfsOrd[i];
            cnt[now]++;
            if (parent[now] != -1) {
                cnt[parent[now]] += cnt[now];
            }
        }
        return new int[][]{ parent, cnt, bfsOrd };
    }


    static int[][] buildGraph(InputReader in, int n, int m) {
        int[][] edges = new int[m][];
        int[][] graph = new int[n][];
        int[] deg = new int[n];
        for (int i = 0 ; i < m ; i++) {
            int a = in.nextInt()-1;
            int b = in.nextInt()-1;
            deg[a]++;
            deg[b]++;
            edges[i] = new int[]{a, b};
        }
        for (int i = 0 ; i < n ; i++) {
            graph[i] = new int[deg[i]];
        }
        for (int i = 0 ; i < m ; i++) {
            int a = edges[i][0];
            int b = edges[i][1];
            graph[a][--deg[a]] = b;
            graph[b][--deg[b]] = a;
        }
        return graph;
    }

    static class InputReader {
        private InputStream stream;
        private byte[] buf = new byte[1024];
        private int curChar;
        private int numChars;

        public InputReader(InputStream stream) {
            this.stream = stream;
        }

        private int[] nextInts(int n) {
            int[] ret = new int[n];
            for (int i = 0; i < n; i++) {
                ret[i] = nextInt();
            }
            return ret;
        }


        private int[][] nextIntTable(int n, int m) {
            int[][] ret = new int[n][m];
            for (int i = 0; i < n; i++) {
                for (int j = 0; j < m; j++) {
                    ret[i][j] = nextInt();
                }
            }
            return ret;
        }

        private long[] nextLongs(int n) {
            long[] ret = new long[n];
            for (int i = 0; i < n; i++) {
                ret[i] = nextLong();
            }
            return ret;
        }

        private long[][] nextLongTable(int n, int m) {
            long[][] ret = new long[n][m];
            for (int i = 0; i < n; i++) {
                for (int j = 0; j < m; j++) {
                    ret[i][j] = nextLong();
                }
            }
            return ret;
        }

        private double[] nextDoubles(int n) {
            double[] ret = new double[n];
            for (int i = 0; i < n; i++) {
                ret[i] = nextDouble();
            }
            return ret;
        }

        private int next() {
            if (numChars == -1)
                throw new InputMismatchException();
            if (curChar >= numChars) {
                curChar = 0;
                try {
                    numChars = stream.read(buf);
                } catch (IOException e) {
                    throw new InputMismatchException();
                }
                if (numChars <= 0)
                    return -1;
            }
            return buf[curChar++];
        }

        public char nextChar() {
            int c = next();
            while (isSpaceChar(c))
                c = next();
            if ('a' <= c && c <= 'z') {
                return (char) c;
            }
            if ('A' <= c && c <= 'Z') {
                return (char) c;
            }
            throw new InputMismatchException();
        }

        public String nextToken() {
            int c = next();
            while (isSpaceChar(c))
                c = next();
            StringBuilder res = new StringBuilder();
            do {
                res.append((char) c);
                c = next();
            } while (!isSpaceChar(c));
            return res.toString();
        }

        public int nextInt() {
            int c = next();
            while (isSpaceChar(c))
                c = next();
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = next();
            }
            int res = 0;
            do {
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                res *= 10;
                res += c-'0';
                c = next();
            } while (!isSpaceChar(c));
            return res*sgn;
        }

        public long nextLong() {
            int c = next();
            while (isSpaceChar(c))
                c = next();
            long sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = next();
            }
            long res = 0;
            do {
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                res *= 10;
                res += c-'0';
                c = next();
            } while (!isSpaceChar(c));
            return res*sgn;
        }

        public double nextDouble() {
            return Double.valueOf(nextToken());
        }

        public boolean isSpaceChar(int c) {
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }

        public interface SpaceCharFilter {
            public boolean isSpaceChar(int ch);
        }
    }

    static void debug(Object... o) {
        System.err.println(Arrays.deepToString(o));
    }
}
