#include <bits/stdc++.h>
#define ff first
#define ss second
#define endl '\n'
using namespace std;
const long long INF = (long long) 1e18;
const int mod = (int) 1e9+7;
const int MAXN = (int) 3e5+5;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
ll n;
ll a[MAXN];
ll pre[MAXN], suf[MAXN];
int main(){
ios_base::sync_with_stdio(false);
cin.tie(nullptr);cout.tie(nullptr);
#ifdef Local
freopen("C:/Users/Admin/Desktop/Yazilim/C/IO/int.txt","r",stdin);
freopen("C:/Users/Admin/Desktop/Yazilim/C/IO/out.txt","w",stdout);
#endif
cin>>n;
for(int i = 1; i <= n; i++){
cin>>a[i];
}
for(int i = 1; i <= n; i++){
pre[i] = pre[i - 1] + max(0LL, a[i - 1] - a[i] + 1);
}
ll ans = INF;
for(int i = n; i >= 0; i--){
suf[i] = suf[i + 1] + max(0LL, a[i + 1] - a[i] + 1);
ans = min(ans, max(suf[i], pre[i]));
}
cout<<ans<<endl;
#ifdef Local
cout<<endl<<fixed<<setprecision(2)<<1000.0 * clock() / CLOCKS_PER_SEC<< " milliseconds ";
#endif
}
