// vaziat meshki-ghermeze !
#include <bits/stdc++.h>
#define pb push_back
#define F first
#define S second
#define all(x) x.begin(), x.end()
#define debug(x) cerr << #x << " : " << x << '\n'
using namespace std;
typedef long long ll;
typedef long double ld;
typedef string str;
typedef pair<ll, ll> pll;
const ll Mod = 1000000007LL;
const int N = 2e5 + 10;
const ll Inf = 2242545357980376863LL;
const ll Log = 30;
int a[N], b[N];
int main(){
ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
int n;
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];
ll sm = 0;
ll sm2 = 0;
for(int i = 1; i < n; i++) if(b[i] >= 0) sm += (b[i] + 1);
ll ans = sm;
for(int i = 1; i < n; i++){
if(b[i] >= 0) sm -= (b[i] + 1);
if(b[i] <= 0) sm2 += (-b[i] + 1);
ans = min(ans, max(sm, sm2));
}
cout << ans << '\n';
return 0;
}
