#include <bits/stdc++.h>
#pragma GCC optimize ("O2,unroll-loops")
//#pragma GCC optimize("no-stack-protector,fast-math")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<int, int> pii;
typedef pair<pii, int> piii;
typedef pair<ll, ll> pll;
#define debug(x) cerr<<#x<<'='<<(x)<<endl;
#define debugp(x) cerr<<#x<<"= {"<<(x.first)<<", "<<(x.second)<<"}"<<endl;
#define debug2(x, y) cerr<<"{"<<#x<<", "<<#y<<"} = {"<<(x)<<", "<<(y)<<"}"<<endl;
#define debugv(v) {cerr<<#v<<" : ";for (auto x:v) cerr<<x<<' ';cerr<<endl;}
#define all(x) x.begin(), x.end()
#define pb push_back
#define kill(x) return cout<<x<<'\n', 0;
const int inf=1000000010;
const ll INF=1000000000000001000LL;
const int mod=1000000007;
const int MAXN=300010, LOG=20;
ll n, m, k, u, v, x, y, t, a, b, ans;
ll A[MAXN], B[MAXN];
ll ps[MAXN][2];
int main(){
ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
//freopen("input.txt", "r", stdin);
//freopen("output.txt", "w", stdout);
cin>>n;
for (int i=1; i<=n; i++) cin>>A[i];
for (int i=1; i<n; i++){
B[i]=A[i+1]-A[i];
ps[i][0]=ps[i-1][0];
ps[i][1]=ps[i-1][1];
if (B[i]<=0) ps[i][0]+=-B[i]+1;
if (B[i]>=0) ps[i][1]+=B[i]+1;
}
ans=INF;
for (int k=1; k<=n; k++){
ll x=ps[k-1][0];
ll y=ps[n-1][1]-ps[k-1][1];
// debug2(x, y)
ans=min(ans, max(x, y));
}
cout<<ans<<"\n";
return 0;
}
