#include <bits/stdc++.h>

#ifndef ONLINE_JUDGE
    #define gc getchar
    #define pc putchar
#else
    #define gc getchar_unlocked
    #define pc putchar_unlocked
#endif

using namespace std;

#define vi vector<int>
#define si set<int>
#define vs vector<string>
#define pii pair<int,int>
#define vpi vector<pii>
#define pri priority_queue<int>
#define rev_pri priority_queue<int,vector<int>,greater<int> >
#define mpi map<int,int>
#define i64 long long int
#define endl '\n'
#define pi acos(-1)
#define all(v) v.begin(),v.end()
#define pb push_back
#define mp make_pair
#define mod 1000000007
#define inf INT_MAX/2
#define infll LLONG_MAX/3
#define For(i,n) for(int i=0;i<n;i++)
#define Fre(i,a,b) for(int i = a; i < b; i++)
#define sf(n) scanf("%d", &n)
#define sff(a,b) scanf("%d %d", &a, &b)
#define sfff(a,b,c) scanf("%d %d %d", &a, &b, &c)
#define pfn(n) printf("%d\n", n)
#define pfs(n) printf("%d ", n)
#define eps 1e-8
#define ff first
#define ss second
#define mem(a,b) memset(a,b,sizeof(a))
#define READ freopen("in.txt", "r", stdin)
#define WRITE freopen("out.txt", "w", stdout)
#define sz size()
#define dbg(i) printf("yo %d\n", i)
#define foreach(i,c) for(__typeof((c).begin()) i = (c).begin(); i != (c).end(); i++)
#define sqr(a) (a) * (a)
#define clr clear()
#define CASE(a) printf("Case %d: ",a)

//int dx[] = {0,1,0,-1,1,1,-1,-1};
//int dy[] = {1,0,-1,0,1,-1,-1,1};


//i64 gcd(i64 a,i64 b){if(!b)return a;return gcd(b,a%b);}

//inline void fastRead(int *a){register char c=0;while(c<33)c=gc();*a=0;while(c>33){*a=*a*10+c-'0';c=gc();}}

//inline void fastWrite(int a){char snum[20];int i=0;do{snum[i++]=a%10+48;a=a/10;}while(a!=0);i=i-1;while(i>=0)pc(snum[i--]);pc('\n');}

//i64 bigmod(i64 num,i64 n){if(n==0)return 1;i64 x=bigmod(num,n/2);x=x*x%mod;if(n%2==1)x=x*num%mod;return x;}

//i64 modinverse(i64 num){return bigmod(num,mod-2)%mod;}

//i64 po(i64 a,i64 b){i64 ans=1;while(b--)ans *= a;return ans;}

//i64 ncr(i64 n,i64 r){if(n==r)return 1;if(r==1)return n;if(dp[n][r]!=-1)return dp[n][r];return dp[n][r]=ncr(n-1,r)+ncr(n-1,r-1);}


// bit manipulations

//bool checkbit(int mask,int bit){return mask & (1<<bit);}

//int setbit(int mask,int bit){ return mask  (1<<bit) ; }

//int clearbit(int mask,int bit){return mask & ~(1<<bit);}

//int togglebit(int mask,int bit){return mask ^ (1<<bit);}

vector<i64> v,w;
int main()
{
        int n,inp;

        sf(n);

        while(n--)
        {
                sf(inp);
                v.pb(inp);
        }

        vi w;
        For(i,v.sz)
        {
                w.pb(v[i] - (i+1));

        }

        sort(all(w));

        i64 rsum = 0;
        int gg = w[0];
        For(i,w.sz)
        {

//                cout << w[i] << " ";
                w[i] -= gg;
//                cout << w[i] << endl;
                rsum += w[i];
        }
//        cout << endl;
        i64 l = 0;
        i64 r = v.sz;
        i64 lsum = 0;
        i64 ans = infll;

//        cout << rsum << endl;
        For(i,w.sz)
        {
                rsum -= w[i];
                r--;
                i64 tem = l * w[i] - lsum + rsum - r * w[i];

//                cout << w[i] << " " << tem << endl;
                ans = min(ans,tem);

                l++;
//                r--;
                lsum += w[i];
//                rsum -= w[i];
        }

        cout << ans << endl;

        return 0;
}




Measured memory_bytes: 3568