#include <iostream>

#include <cstdio>

#include <cstring>

#include <string>

#include <algorithm>

#include <vector>

#include <queue>

#include <stack>

#include <map>

#include <set>

#include <cmath>

using namespace std;

int n,m;

int a[100010];

bool cmp(int x,int y)

{

	return x>y;

}

int main()

{

	scanf("%d%d",&n,&m);

	for(int i=1;i<=m;i++) scanf("%d",&a[i]);

	sort(a+1,a+1+m,cmp);

	int now = n;

	if(n==0||n==1)

	{

		printf("0\n");

		return 0;

	}

	for(int i=1;i<=m;i++)

	{

		if(a[i] <= 0) break;

		if(a[i]<now)

		{

			now = now - a[i];

			a[i+1]--;

		}

		else

		{

			now = 0;

			printf("%d\n",i);

			break;

		}

	}

	if(now!=0)

	{

		printf("Impossible\n");

	}

	return 0;

}