#include<cstdio>

#include<algorithm>

#include<cstring>

using namespace std;

const int maxn=,inf=0x3f3f3f3f;

int dp[maxn][maxn],f[maxn],sum[maxn][maxn],ms[maxn][maxn],n,m,A[maxn];

int calc(int a,int b,int c,int d)

{

    int m1=ms[a][b],m2=ms[c][d];

    int ans=inf;

    ans=(b-a+)-(sum[b][m2]-sum[a-][m2])+(d-c+);

    ans=min(ans,(d-c+)-(sum[d][m1]-sum[c-][m1])+(b-a+));

    return ans;

}

bool B[maxn];

bool mex(int a,int b)

{

    memset(B,,sizeof(B));

    for(int i=a;i<=b;i++)if(B[A[i]])return ;else B[A[i]]++;

    bool ok=;

    if(!B[])return ;

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

    {

        if(ok&&B[i])return ;

        if(B[i-]&&!B[i])ok=;

    }

    return ;

}

int main()

{

    scanf("%d",&n);

    m=;

    for(int i=;i<=n;i++){scanf("%d",&A[i]);m=max(m,A[i]+);}

    for(int i=;i<=n;i++)

    {

        for(int j=;j<=A[i];j++)sum[i][j]=sum[i-][j];

        for(int j=A[i];j<m;j++)sum[i][j]=sum[i-][j]+;

        ms[i][i]=A[i];

        for(int j=i+;j<=n;j++)ms[i][j]=min(ms[i][j-],A[j]);

    }

    for(int p=;p<=n-;p++)

    {

        for(int i=;i<=n-p;i++)//

        {

            int j=i+p;

            dp[i][j]=inf;

            for(int k=i;k<j;k++)if(dp[i][k]<inf&&dp[k+][j]<inf)//

             dp[i][j]=min(dp[i][j],dp[i][k]+dp[k+][j]+calc(i,k,k+,j));

        }

    }

    memset(f,0x3f,sizeof(f));

    f[]=;//初始化！！！

    for(int i=;i<=n;i++)

    {

        for(int j=;j<i;j++)if(f[j]<inf-)

        {

            if(mex(j+,i))

            {

                f[i]=min(f[i],f[j]+dp[j+][i]);

            }

        }

    }

    if(f[n]>inf-)printf("Impossible");

    else printf("%d",f[n]);

    return ;

}