//#pragma optimization_level 3
//#pragma GCC optimize("Ofast,no-stack-protector,unroll-loops,fast-math,O3")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#include<bits/stdc++.h>
/*
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/detail/standard_policies.hpp>
using namespace __gnu_pbds;
typedef tree<int,null_type,less<int>,rb_tree_tag,tree_order_statistics_node_update>ordset;
*/
#define fr first
#define sc second
#define vec vector
#define pb push_back
#define pii pair<int, int>
#define forn(x,y) for(int x = 1 ; x <= (int)y ; ++x)
#define all(x) (x).begin(),(x).end()
#define fast cin.tie(0);cout.tie(0);cin.sync_with_stdio(0);cout.sync_with_stdio(0);
using namespace std;
typedef long long ll;
typedef unsigned int uint;
typedef pair<ll,ll> pll;
typedef complex<int> point;
const int nmax = 400005;
const ll linf = 1e18;
const ll mod = 998244353;
const int inf = 1e9;
const int sq = 5000;
int n, k;
int a[nmax],b[nmax];
int main(){
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];
}
// b[i] > 0 , b[i] < 0
ll cnt1(0), cnt2(0);
for(int i = 1; i < n; i++){
cnt2 += max(0,b[i]+1);
}
ll rs = max(cnt1,cnt2);
for(int k = 1; k < n; k++){
cnt1 += max(0,1-b[k]);
cnt2 -= max(0,b[k]+1);
//cout << k << ' ' << cnt1 << ' ' << cnt2 << '\n';
rs = min(rs, max(cnt1,cnt2));
}
cout << rs << '\n';
}
