import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.util.Arrays;
import java.io.IOException;
import java.util.ArrayList;
import java.io.UncheckedIOException;
import java.util.List;
import java.io.Closeable;
import java.io.Writer;
import java.io.OutputStreamWriter;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 */
public class Main {
    public static void main(String[] args) throws Exception {
        Thread thread = new Thread(null, new TaskAdapter(), "", 1 << 27);
        thread.start();
        thread.join();
    }

    static class TaskAdapter implements Runnable {
        @Override
        public void run() {
            InputStream inputStream = System.in;
            OutputStream outputStream = System.out;
            FastInput in = new FastInput(inputStream);
            FastOutput out = new FastOutput(outputStream);
            TaskE solver = new TaskE();
            solver.solve(1, in, out);
            out.close();
        }
    }

    static class TaskE {
        boolean[][] next;
        int n;
        int[] colors;
        boolean valid = true;

        public void solve(int testNumber, FastInput in, FastOutput out) {
            n = in.readInt();
            int m = in.readInt();
            next = new boolean[n][n];
            colors = new int[n];
            SequenceUtils.deepFill(next, true);
            SequenceUtils.deepFill(colors, -1);
            for (int i = 0; i < m; i++) {
                int a = in.readInt() - 1;
                int b = in.readInt() - 1;
                next[a][b] = next[b][a] = false;
            }

            List<int[]> cnts = new ArrayList<>(n);
            for (int i = 0; i < n; i++) {
                if (colors[i] != -1) {
                    continue;
                }
                int[] c = new int[2];
                dfs(i, 0, c);
                cnts.add(c);
            }

            if (!valid) {
                out.println(-1);
                return;
            }

            int k = cnts.size();
            boolean[][] dp = new boolean[k + 1][n / 2 + 1];
            dp[0][0] = true;
            for (int i = 1; i <= k; i++) {
                int[] c = cnts.get(i - 1);
                for (int j = 0; j <= n / 2; j++) {
                    if (j >= c[0]) {
                        dp[i][j] = dp[i][j] || dp[i - 1][j - c[0]];
                    }
                    if (j >= c[1]) {
                        dp[i][j] = dp[i][j] || dp[i - 1][j - c[1]];
                    }
                }
            }

            int mx = 0;
            for (int i = 0; i <= n / 2; i++) {
                if (dp[k][i]) {
                    mx = i;
                }
            }

            out.println(pick2(mx) + pick2(n - mx));
        }

        public int pick2(int n) {
            return n * (n - 1) / 2;
        }

        public void dfs(int root, int color, int[] cnts) {
            if (colors[root] != -1) {
                if (color != colors[root]) {
                    valid = false;
                }
                return;
            }
            colors[root] = color;
            cnts[color]++;
            for (int i = 0; i < n; i++) {
                if (!next[root][i] || i == root) {
                    continue;
                }
                dfs(i, color ^ 1, cnts);
            }
        }

    }

    static class FastOutput implements AutoCloseable, Closeable, Appendable {
        private StringBuilder cache = new StringBuilder(10 << 20);
        private final Writer os;

        public FastOutput append(CharSequence csq) {
            cache.append(csq);
            return this;
        }

        public FastOutput append(CharSequence csq, int start, int end) {
            cache.append(csq, start, end);
            return this;
        }

        public FastOutput(Writer os) {
            this.os = os;
        }

        public FastOutput(OutputStream os) {
            this(new OutputStreamWriter(os));
        }

        public FastOutput append(char c) {
            cache.append(c);
            return this;
        }

        public FastOutput append(int c) {
            cache.append(c);
            return this;
        }

        public FastOutput println(int c) {
            return append(c).println();
        }

        public FastOutput println() {
            cache.append(System.lineSeparator());
            return this;
        }

        public FastOutput flush() {
            try {
                os.append(cache);
                os.flush();
                cache.setLength(0);
            } catch (IOException e) {
                throw new UncheckedIOException(e);
            }
            return this;
        }

        public void close() {
            flush();
            try {
                os.close();
            } catch (IOException e) {
                throw new UncheckedIOException(e);
            }
        }

        public String toString() {
            return cache.toString();
        }

    }

    static class SequenceUtils {
        public static void deepFill(Object array, int val) {
            if (!array.getClass().isArray()) {
                throw new IllegalArgumentException();
            }
            if (array instanceof int[]) {
                int[] intArray = (int[]) array;
                Arrays.fill(intArray, val);
            } else {
                Object[] objArray = (Object[]) array;
                for (Object obj : objArray) {
                    deepFill(obj, val);
                }
            }
        }

        public static void deepFill(Object array, boolean val) {
            if (!array.getClass().isArray()) {
                throw new IllegalArgumentException();
            }
            if (array instanceof boolean[]) {
                boolean[] intArray = (boolean[]) array;
                Arrays.fill(intArray, val);
            } else {
                Object[] objArray = (Object[]) array;
                for (Object obj : objArray) {
                    deepFill(obj, val);
                }
            }
        }

    }

    static class FastInput {
        private final InputStream is;
        private byte[] buf = new byte[1 << 20];
        private int bufLen;
        private int bufOffset;
        private int next;

        public FastInput(InputStream is) {
            this.is = is;
        }

        private int read() {
            while (bufLen == bufOffset) {
                bufOffset = 0;
                try {
                    bufLen = is.read(buf);
                } catch (IOException e) {
                    bufLen = -1;
                }
                if (bufLen == -1) {
                    return -1;
                }
            }
            return buf[bufOffset++];
        }

        public void skipBlank() {
            while (next >= 0 && next <= 32) {
                next = read();
            }
        }

        public int readInt() {
            int sign = 1;

            skipBlank();
            if (next == '+' || next == '-') {
                sign = next == '+' ? 1 : -1;
                next = read();
            }

            int val = 0;
            if (sign == 1) {
                while (next >= '0' && next <= '9') {
                    val = val * 10 + next - '0';
                    next = read();
                }
            } else {
                while (next >= '0' && next <= '9') {
                    val = val * 10 - next + '0';
                    next = read();
                }
            }

            return val;
        }

    }
}



Measured memory_bytes: 51260