#include<bits/stdc++.h>
using namespace std;
#define S second
#define F first
#define ll long long
//#define int long long
//#pragma GCC optimize("Ofast, unroll-loop")
//#pragma GCC target("avx,avx2")
#pragma GCC optimize("O3")
const int inf=0x3f3f3f3f;
const ll inff=0x3f3f3f3f3f3f3f3f;
const int X=1000000007;
int a[200005], d[200005];
ll pos[200005], neg[200005], ans=inff;
signed main(){
ios::sync_with_stdio(0), cin.tie(0);
int n; cin >> n;
for(int i=1 ; i<=n ; i++) cin >> a[i];
for(int i=1 ; i<=n ; i++) d[i]=a[i]-a[i-1];
for(int i=1 ; i<=n ; i++)
pos[i]=pos[i-1]+(d[i]<=0 ? 1-d[i] : 0),
neg[i]=neg[i-1]+(d[i]>=0 ? d[i]+1 : 0);
for(int i=1 ; i<=n ; i++){
ans=min(ans,max(pos[i],neg[n]-neg[i]));
}
cout << ans << '\n';
return 0;
}
