Problem:
Score : 300 points

Problem Statement

There is a staircase with N steps. Takahashi is now standing at the foot of the stairs, that is, on the 0-th step.
He can climb up one or two steps at a time.

However, the treads of the a_1-th, a_2-th, a_3-th, \ldots, a_M-th steps are broken, so it is dangerous to set foot on those steps.

How many are there to climb up to the top step, that is, the N-th step, without setting foot on the broken steps?
Find the count modulo 1\ 000\ 000\ 007.

Constraints

1 \leq N \leq 10^5

0 \leq M \leq N-1

1 \leq a_1 < a_2 < ... < a_M \leq N-1

Input

Input is given from Standard Input in the following format:

N M
a_1
a_2
.
.
.
a_M

Output

Print the number of ways to climb up the stairs under the condition, modulo 1\ 000\ 000\ 007.

Sample Input 1

6 1
3

Sample Output 1

4

There are four ways to climb up the stairs, as follows:

0 \to 1 \to 2 \to 4 \to 5 \to 6

0 \to 1 \to 2 \to 4 \to 6

0 \to 2 \to 4 \to 5 \to 6

0 \to 2 \to 4 \to 6

Sample Input 2

10 2
4
5

Sample Output 2

0

There may be no way to climb up the stairs without setting foot on the broken steps.

Sample Input 3

100 5
1
23
45
67
89

Sample Output 3

608200469

Be sure to print the count modulo 1\ 000\ 000\ 007.

Solution in Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.math.BigDecimal;
import java.text.DecimalFormat;
import java.util.AbstractMap;
import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Comparator;
import java.util.Deque;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.PriorityQueue;
import java.util.Queue;
import java.util.Set;
import java.util.TreeMap;
import java.util.TreeSet;

import static java.util.Comparator.*;				

public class Main {
	
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        MyInput in = new MyInput(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        
        Solver solver = new Solver(in, out);
        solver.solve();
        
        out.close();
    }
    
    // ======================================================================

    static class Solver {
    	MyInput in;
    	PrintWriter out;
        public Solver(MyInput in, PrintWriter out) {
        	this.in = in;
        	this.out = out;
        }
        // -----------------------------------------
        int MOD = 1000000007;
    	long[] dp;
        public void solve() {
        	int N = ni();
        	int M = ni();
        	int[] A = ndi(M);
        	dp = new long[N+2];
        	dp[0] = dp[1] = 1;
        	for(int i=0; i < M; i++) {
        		dp[A[i]] = -1;
        	}
        	for(int i = 0; i <= N; i++) {
        		if(dp[i] == -1) {
        			dp[i] = 0;
//        			prn("dp[" + i + "] = " + dp[i]);
        		} else if(i >= 2){
        			dp[i] = (dp[i-2] + dp[i-1]) % MOD;
//        			prn("dp[" + i + "] = " + dp[i]);
        		}
        	}
        	prn(dp[N]);
        }
        
        // -----------------------------------------
        void prn(String s) {
        	out.println(s);
        }
        void prn(int i) {
        	out.println(i);
        }
        void prn(long i) {
        	out.println(i);
        }
        void prr(String s) {
        	out.print(s);
        }
        int ni() {
        	return in.nextInt();
        }
        long nl() {
        	return in.nextLong();
        }
        String ns() {
        	return in.nextString();
        }
        int[] ndi(int n) {
        	int[] ans = new int[n];
        	for(int i=0; i < n; i++) {
        		ans[i] = ni();
        	}
        	return ans;
        }
        long[] ndl(int n) {
        	long[] ans = new long[n];
        	for(int i=0; i < n; i++) {
        		ans[i] = nl();
        	}
        	return ans;
        }
        String[] nds(int n) {
        	String[] ans = new String[n];
        	for(int i=0; i < n; i++) {
        		ans[i] = ns();
        	}
        	return ans;
        }
        int[][] nddi(int n, int m) {
        	int[][] ans = new int[n][m];
        	for(int i=0; i < n; i++) {
            	for(int j=0; j < m; j++) {
            		ans[i][j] = ni();
            	}
        	}
        	return ans;
        }
        long[][] nddl(int n, int m) {
        	long[][] ans = new long[n][m];
        	for(int i=0; i < n; i++) {
            	for(int j=0; j < m; j++) {
            		ans[i][j] = nl();
            	}
        	}
        	return ans;
        }
    }

    static class PP{
    	public int key, val;
    	public PP(int key, int val) {
    		this.key = key;
    		this.val = val;
    	}
		public int getKey() {
			return key;
		}
		public void setKey(int key) {
			this.key = key;
		}
		public int getVal() {
			return val;
		}
		public void setVal(int val) {
			this.val = val;
		}
    }
    static class PPL {
    	public long key, val;
    	public PPL(long key, long val) {
    		this.key = key;
    		this.val = val;
    	}
		public long getKey() {
			return key;
		}
		public void setKey(long key) {
			this.key = key;
		}
		public long getVal() {
			return val;
		}
		public void setVal(long val) {
			this.val = val;
		}
    }
    static class PPDL {
    	public long key;
    	public long[] val;
    	public PPDL(long key, long[] val) {
    		this.key = key;
    		this.val = val;
    	}
		public long getKey() {
			return key;
		}
		public void setKey(long key) {
			this.key = key;
		}
		public long[] getVal() {
			return val;
		}
		public void setVal(long[] val) {
			this.val = val;
		}
		public void dump(PrintWriter out) {
			out.print("key = " + key + "  val ");
			for(int i=0; i < val.length; i++) {
				out.print("[" + val[i] + "] ");
			}
			out.println("");
		}
    }
    // ======================================================================
    static class Pair<K, V> extends AbstractMap.SimpleEntry<K, V> {
        /** serialVersionUID. */
        private static final long serialVersionUID = 6411527075103472113L;

        public Pair(final K key, final V value) {
            super(key, value);
        }
     }
    static class MyInput {
        private final BufferedReader in;
        private static int pos;
        private static int readLen;
        private static final char[] buffer = new char[1024 * 8];
        private static char[] str = new char[500 * 8 * 2];
        private static boolean[] isDigit = new boolean[256];
        private static boolean[] isSpace = new boolean[256];
        private static boolean[] isLineSep = new boolean[256];
 
        static {
            for (int i = 0; i < 10; i++) {
                isDigit['0' + i] = true;
            }
            isDigit['-'] = true;
            isSpace[' '] = isSpace['\r'] = isSpace['\n'] = isSpace['\t'] = true;
            isLineSep['\r'] = isLineSep['\n'] = true;
        }
 
        public MyInput(InputStream is) {
            in = new BufferedReader(new InputStreamReader(is));
        }
 
        public int read() {
            if (pos >= readLen) {
                pos = 0;
                try {
                    readLen = in.read(buffer);
                } catch (IOException e) {
                    throw new RuntimeException();
                }
                if (readLen <= 0) {
                    throw new MyInput.EndOfFileRuntimeException();
                }
            }
            return buffer[pos++];
        }
 
        public int nextInt() {
            int len = 0;
            str[len++] = nextChar();
            len = reads(len, isSpace);
            int i = 0;
            int ret = 0;
            if (str[0] == '-') {
                i = 1;
            }
            for (; i < len; i++) ret = ret * 10 + str[i] - '0';
            if (str[0] == '-') {
                ret = -ret;
            }
            return ret;
        }
 
        public long nextLong() {
            int len = 0;
            str[len++] = nextChar();
            len = reads(len, isSpace);
            int i = 0;
            long ret = 0L;
            if (str[0] == '-') {
                i = 1;
            }
            for (; i < len; i++) ret = ret * 10 + str[i] - '0';
            if (str[0] == '-') {
                ret = -ret;
            }
            return ret;
        }
 
       public String nextString() {
        	String ret = new String(nextDChar()).trim();
            return ret;
        }
 
        public char[] nextDChar() {
            int len = 0;
            len = reads(len, isSpace);
            char[] ret = new char[len + 1];
            for (int i=0; i < len; i++)		ret[i] = str[i];
            ret[len] = 0x00;
            return ret;
        }
 
        public char nextChar() {
            while (true) {
                final int c = read();
                if (!isSpace[c]) {
                    return (char) c;
                }
            }
        }
 
        int reads(int len, boolean[] accept) {
            try {
                while (true) {
                    final int c = read();
                    if (accept[c]) {
                        break;
                    }
                    if (str.length == len) {
                        char[] rep = new char[str.length * 3 / 2];
                        System.arraycopy(str, 0, rep, 0, str.length);
                        str = rep;
                    }
                    str[len++] = (char) c;
                }
            } catch (MyInput.EndOfFileRuntimeException e) {
            }
            return len;
        }
 
        static class EndOfFileRuntimeException extends RuntimeException {
        }
    }
}