import java.util.*;
import java.io.*;

public class Main {
    public static void solve(ContestScanner sc, ContestPrinter ou) {
        int n = sc.nextInt();
        int k = sc.nextInt() - 1;
        var a = sc.nextIntArray(n);
        if (n == 1)
            ou.println("1/1");
        else {
            long l = 0, r = 0;
            for (int i = 0; i < n - 1; i++) {
                if (i < k)
                    l += 3600 / a[i] / a[i + 1];
                r += 3600 / a[i] / a[i + 1];
            }
            if (l == 0)
                ou.println(0);
            else {
                long s = g(l, r);
                l /= s;
                r /= s;
                ou.println(l + "/" + r);
            }
        }
    }

    static long g(long a, long b) {
        return b == 0 ? a : g(b, a % b);
    }

    public static void main(String[] args) {
        var sc = new ContestScanner();
        var ou = new ContestPrinter();
        solve(sc, ou);
        ou.flush();
        ou.close();
    }

    public static int intArray(int[] a, java.util.function.IntBinaryOperator map) {
        int s = a[0];
        for (int i = 1; i < a.length; i++)
            s = map.applyAsInt(s, a[i]);
        return s;
    }

    public static long longArray(long[] a, java.util.function.LongBinaryOperator map) {
        long s = a[0];
        for (int i = 1; i < a.length; i++)
            s = map.applyAsLong(s, a[i]);
        return s;
    }

    public static int max(int s, int... a) {
        for (int i : a)
            if (s < i)
                s = i;
        return s;
    }

    public static long max(long s, long... a) {
        for (long i : a)
            if (s < i)
                s = i;
        return s;
    }

    public static int min(int s, int... a) {
        for (int i : a)
            if (s > i)
                s = i;
        return s;
    }

    public static long min(long s, long... a) {
        for (long i : a)
            if (s > i)
                s = i;
        return s;
    }

    static class Point {
        int x;
        int y;

        Point(int x, int y) {
            this.x = x;
            this.y = y;
        }

        int compareTo(Point p) {
            long c = this.x - p.x;
            if (c < 0)
                return -1;
            if (c > 0)
                return 1;
            return 0;
        }

        boolean equals(Point p) {
            return this.x == p.x && this.y == p.y;
        }
    }

    static class Point2 {
        long x;
        int y;

        Point2(long x, int y) {
            this.x = x;
            this.y = y;
        }

        int compareTo(Point2 p) {
            long c = this.x - p.x;
            if (c < 0)
                return -1;
            if (c > 0)
                return 1;
            return 0;
        }

        boolean equals(Point2 p) {
            return this.x == p.x && this.y == p.y;
        }
    }
}

class ContestScanner {
    private final InputStream in;
    private final byte[] buffer = new byte[1024];
    private int ptr = 0;
    private int buflen = 0;

    public ContestScanner(InputStream in) {
        this.in = in;
    }

    public ContestScanner() {
        this(System.in);
    }

    private boolean hasNextByte() {
        if (ptr < buflen)
            return true;
        ptr = 0;
        try {
            buflen = in.read(buffer);
        } catch (IOException e) {
            e.printStackTrace();
        }
        if (buflen <= 0)
            return false;
        return true;
    }

    private int readByte() {
        return hasNextByte() ? buffer[ptr++] : -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 void nextThrow(int n) {
        for (int i = 0; i < n; i++)
            this.next();
    }

    public void nextThrow() {
        this.nextThrow(1);
    }

    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 boolean[] nextBoolean(char True) {
        String s = this.next();
        int n = s.length();
        boolean[] array = new boolean[n];
        for (int i = 0; i < n; i++)
            array[i] = s.charAt(i) == True;
        return array;
    }

    public long[] nextLongArray(int length) {
        long[] array = new long[length];
        for (int i = 0; i < length; i++)
            array[i] = this.nextLong();
        return array;
    }

    public long[] nextLongArray(int length, java.util.function.LongUnaryOperator map) {
        long[] array = new long[length];
        for (int i = 0; i < length; i++)
            array[i] = map.applyAsLong(this.nextLong());
        return array;
    }

    public int[] nextIntArray(int length) {
        int[] array = new int[length];
        for (int i = 0; i < length; i++)
            array[i] = this.nextInt();
        return array;
    }

    public int[] nextIntArray(int length, java.util.function.IntUnaryOperator map) {
        int[] array = new int[length];
        for (int i = 0; i < length; i++)
            array[i] = map.applyAsInt(this.nextInt());
        return array;
    }

    public int[] nextIntArray(int length, int[] array) {
        int n = length + array.length;
        int[] a = new int[n];
        for (int i = 0; i < length; i++)
            a[i] = this.nextInt();
        for (int i = length; i < n; i++)
            a[i] = array[i - length];
        return a;
    }

    public Integer[] nextIntegerArray(int length, java.util.function.IntUnaryOperator map) {
        Integer[] array = new Integer[length];
        for (int i = 0; i < length; i++)
            array[i] = map.applyAsInt(this.nextInt());
        return array;
    }

    public Integer[] nextIntegerArray(int length) {
        Integer[] array = new Integer[length];
        for (int i = 0; i < length; i++)
            array[i] = this.nextInt();
        return array;
    }

    public double[] nextDoubleArray(int length) {
        double[] array = new double[length];
        for (int i = 0; i < length; i++)
            array[i] = this.nextDouble();
        return array;
    }

    public double[] nextDoubleArray(int length, java.util.function.DoubleUnaryOperator map) {
        double[] array = new double[length];
        for (int i = 0; i < length; i++)
            array[i] = map.applyAsDouble(this.nextDouble());
        return array;
    }

    public String[] nextArray(int length) {
        String[] array = new String[length];
        for (int i = 0; i < length; i++)
            array[i] = this.next();
        return array;
    }

    public long[][] nextLongMatrix(int height, int width) {
        long[][] mat = new long[height][width];
        for (int h = 0; h < height; h++)
            for (int w = 0; w < width; w++)
                mat[h][w] = this.nextLong();
        return mat;
    }

    public int[][] nextIntMatrix(int height, int width) {
        int[][] mat = new int[height][width];
        for (int h = 0; h < height; h++)
            for (int w = 0; w < width; w++)
                mat[h][w] = this.nextInt();
        return mat;
    }

    public double[][] nextDoubleMatrix(int height, int width) {
        double[][] mat = new double[height][width];
        for (int h = 0; h < height; h++)
            for (int w = 0; w < width; w++)
                mat[h][w] = this.nextDouble();
        return mat;
    }

    public boolean[][] nextBooleanMatrix(int height, int width, char True) {
        boolean[][] mat = new boolean[height][width];
        for (int h = 0; h < height; h++) {
            String s = this.next();
            for (int w = 0; w < width; w++)
                mat[h][w] = s.charAt(w) == True;
        }
        return mat;
    }

    public char[][] nextCharMatrix(int height, int width) {
        char[][] mat = new char[height][width];
        for (int h = 0; h < height; h++) {
            String s = this.next();
            for (int w = 0; w < width; w++)
                mat[h][w] = s.charAt(w);
        }
        return mat;
    }

    public char[][] nextCharMatrix(int height, int width, int h, int w, char c) {
        char[][] mat = new char[height + 2 * h][width + 2 * w];
        for (int i = 0; i < height; i++) {
            String s = this.next();
            for (int j = 0; j < width; j++)
                mat[i + h][j + w] = s.charAt(j);
        }
        for (int i = 0; i < h; i++)
            for (int j = 0; j < 2 * w + width; j++)
                mat[i][j] = c;
        for (int i = h + height; i < 2 * h + height; i++)
            for (int j = 0; j < 2 * w + width; j++)
                mat[i][j] = c;
        for (int i = h; i < h + height; i++) {
            for (int j = 0; j < w; j++)
                mat[i][j] = c;
            for (int j = w + width; j < 2 * w + width; j++)
                mat[i][j] = c;
        }
        return mat;
    }

    public boolean[][] nextBooleanMatrix(int height, int width, int h, int w, char c) {
        boolean[][] mat = new boolean[height + 2 * h][width + 2 * w];
        for (int i = 0; i < height; i++) {
            String s = this.next();
            for (int j = 0; j < width; j++)
                mat[i + h][j + w] = s.charAt(j) == c;
        }
        return mat;
    }
}

class ContestPrinter extends PrintWriter {
    public ContestPrinter(PrintStream stream) {
        super(stream);
    }

    public ContestPrinter() {
        super(System.out);
    }

    private static String dtos(double x, int n) {
        StringBuilder sb = new StringBuilder();
        if (x < 0) {
            sb.append('-');
            x = -x;
        }
        x += Math.pow(10, -n) / 2;
        sb.append((long) x);
        sb.append(".");
        x -= (long) x;
        for (int i = 0; i < n; i++) {
            x *= 10;
            sb.append((int) x);
            x -= (int) x;
        }
        return sb.toString();
    }

    @Override
    public void print(float f) {
        super.print(dtos(f, 20));
    }

    @Override
    public void println(float f) {
        super.println(dtos(f, 20));
    }

    @Override
    public void print(double d) {
        super.print(dtos(d, 20));
    }

    @Override
    public void println(double d) {
        super.println(dtos(d, 20));
    }

    public void printlnArray(String[] array) {
        for (String i : array)
            super.println(i);
    }

    public void printArray(int[] array, String separator) {
        int n = array.length - 1;
        for (int i = 0; i < n; i++) {
            super.print(array[i]);
            super.print(separator);
        }
        super.println(array[n]);
    }

    public void printArray(int[] array) {
        this.printArray(array, " ");
    }

    public void printArray(Integer[] array) {
        this.printArray(array, " ");
    }

    public void printArray(Integer[] array, String separator) {
        int n = array.length - 1;
        for (int i = 0; i < n; i++) {
            super.print(array[i]);
            super.print(separator);
        }
        super.println(array[n]);
    }

    public void printlnArray(int[] array) {
        for (int i : array)
            super.println(i);
    }

    public void printArray(int[] array, String separator, java.util.function.IntUnaryOperator map) {
        int n = array.length - 1;
        for (int i = 0; i < n; i++) {
            super.print(map.applyAsInt(array[i]));
            super.print(separator);
        }
        super.println(map.applyAsInt(array[n]));
    }

    public void printlnArray(int[] array, java.util.function.IntUnaryOperator map) {
        for (int i : array)
            super.println(map.applyAsInt(i));
    }

    public void printlnArray(long[] array, java.util.function.LongUnaryOperator map) {
        for (long i : array)
            super.println(map.applyAsLong(i));
    }

    public void printArray(int[] array, java.util.function.IntUnaryOperator map) {
        this.printArray(array, " ", map);
    }

    public void printArray(long[] array, String separator) {
        int n = array.length - 1;
        for (int i = 0; i < n; i++) {
            super.print(array[i]);
            super.print(separator);
        }
        super.println(array[n]);
    }

    public void printArray(long[] array) {
        this.printArray(array, " ");
    }

    public void printlnArray(long[] array) {
        for (long i : array)
            super.println(i);
    }

    public void printArray(boolean[] array, String a, String b) {
        int n = array.length - 1;
        for (int i = 0; i < n; i++)
            super.print((array[i] ? a : b) + " ");
        super.println(array[n] ? a : b);
    }

    public void printArray(boolean[] array) {
        this.printArray(array, "Y", "N");
    }

    public void printArray(long[] array, String separator, java.util.function.LongUnaryOperator map) {
        int n = array.length - 1;
        for (int i = 0; i < n; i++) {
            super.print(map.applyAsLong(array[i]));
            super.print(separator);
        }
        super.println(map.applyAsLong(array[n]));
    }

    public void printArray(long[] array, java.util.function.LongUnaryOperator map) {
        this.printArray(array, " ", map);
    }

    public void printArray(ArrayList<?> array) {
        this.printArray(array, " ");
    }

    public void printArray(ArrayList<?> array, String separator) {
        int n = array.size() - 1;
        for (int i = 0; i < n; i++) {
            super.print(array.get(i).toString());
            super.print(separator);
        }
        super.println(array.get(n).toString());
    }

    public void printlnArray(ArrayList<?> array) {
        int n = array.size();
        for (int i = 0; i < n; i++)
            super.println(array.get(i).toString());
    }

    public void printArray(int[][] array) {
        int n = array.length;
        if (n == 0)
            return;
        int m = array[0].length - 1;
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < m; j++)
                super.print(array[i][j] + " ");
            super.println(array[i][m]);
        }
    }

    public void printArray(long[][] array) {
        int n = array.length;
        if (n == 0)
            return;
        int m = array[0].length - 1;
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < m; j++)
                super.print(array[i][j] + " ");
            super.println(array[i][m]);
        }
    }
}