#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <deque>
#include <cmath>
#include <climits>
#include <cstring>
#include <iomanip>
#include <numeric>
#include <functional>
#include <bitset>
#include <unordered_map>
#include <unordered_set>
#include <chrono>
#include <random>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
#define rep(a,b) for(int a = 0;a<b;a++)
#define st first
#define nd second
#define pb push_back
#define all(a) a.begin(),a.end()
const int inf = 1e9;
const ll infl = 1e18;
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
int n;
cin >> n;
vector<ll> a(n);
rep(i,n) cin >> a[i];
vector<ll> pref(n-1);
rep(i,n-1) pref[i] = a[i+1]-a[i];
ll w = inf;
ll akt = 0;
rep(i,n-1) {
if(pref[i]>=0) {
akt += pref[i]+1;
}
}
w = min(w,akt);
akt = 0;
rep(i, n-1) {
if(pref[i] <= 0) {
akt += -pref[i]+1;
}
}
w = min(w,akt);
vector<ll> ile_dod(n-1);
vector<ll> ile_odj(n-1);
rep(i,n-1){
if(i == 0){
if(pref[i] <= 0){
ile_dod[i] = -pref[i]+1;
}
}
else if(pref[i] <= 0){
ile_dod[i] = ile_dod[i-1] -pref[i]+1;
}
else{
ile_dod[i] = ile_dod[i-1];
}
}
for(int i = n-2; i >= 0; i--){
if(i == n-2){
if(pref[i] >= 0){
ile_odj[i] = pref[i]+1;
}
}
else if(pref[i] >= 0){
ile_odj[i] = ile_odj[i+1]+pref[i]+1;
}
else{
ile_odj[i] = ile_odj[i+1];
}
}
for(int i = 0; i < n-2; i++){
ll akt2 = max(ile_dod[i], ile_odj[i+1]);
w = min(akt2, w);
}
cout << w << "\n";
return 0;
}
