/**************************************************************

     Problem: 1049

     User: Tunix

     Language: C++

     Result: Accepted

     Time:260 ms

     Memory:2912 kb

 ****************************************************************/

 //BZOJ 1049

 #include<vector>

 #include<cstdio>

 #include<cstring>

 #include<cstdlib>

 #include<iostream>

 #include<algorithm>

 #define rep(i,n) for(int i=0;i<n;++i)

 #define F(i,j,n) for(int i=j;i<=n;++i)

 #define D(i,j,n) for(int i=j;i>=n;--i)

 #define pb push_back

 using namespace std;

 inline int getint(){

     int v=,sign=; char ch=getchar();

     while(ch<''||ch>''){ if (ch=='-') sign=-; ch=getchar();}

     while(ch>=''&&ch<=''){ v=v*+ch-''; ch=getchar();}

     return v*sign;

 }

 const int N=,INF=~0u>>;

 typedef long long LL;

 /******************tamplate*********************/

 int head[N],to[N],next[N],cnt;

 void ins(int x,int y){

     to[++cnt]=y; next[cnt]=head[x]; head[x]=cnt;

 }

 int n,a[N],b[N],f[N],len;

 int Find(int x){

     int l=,r=len,mid,ans=;

     while(l<=r){

         mid=l+r>>;

         if (b[mid]<=x) ans=mid,l=mid+;

         else r=mid-;

     }

     return ans+;

 }

 LL g[N],sum1[N],sum2[N];

 void solve2(){

     D(i,n,){

         ins(f[i],i);

         g[i]=1LL<<;

     }

     g[]=;

     a[]=-<<;//这句必须有，不然在BZOJ上会WA……

     F(x,,n)

         for(int i=head[f[x]-];i;i=next[i]){//枚举从哪个地方转移过来

             if (to[i]>x) break;

             if (a[to[i]]>a[x]) continue;

             for(int k=to[i];k<=x;k++)

                 sum1[k]=abs(a[k]-a[to[i]]),sum2[k]=abs(a[x]-a[k]);

             for(int k=to[i]+;k<=x;k++)

                 sum1[k]+=sum1[k-],sum2[k]+=sum2[k-];

             for(int k=to[i];k<x;k++)

                 g[x]=min(g[x],g[to[i]]+sum1[k]-sum1[to[i]]+sum2[x]-sum2[k]);

         }

     cout <<g[n]<<endl;

 }

 int main(){

     n=getint();

     F(i,,n) a[i]=getint()-i;

     a[++n]=<<;

     F(i,,n){

         int x=Find(a[i]);

         f[i]=x; b[x]=a[i];

         if (x>len) len=x;

     }

 //  F(i,1,n) printf("%d ",f[i]);puts("");

     printf("%d\n",n-f[n]);//task 1 end

     solve2();

     return ;

 }