#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define ull unsigned long long
#define pb(e) push_back(e)
#define sv(a) sort(a.begin(),a.end())
#define sa(a,n) sort(a,a+n)
#define mp(a,b) make_pair(a,b)
#define vf first
#define vs second
#define ar array
#define all(x) x.begin(),x.end()
const int inf = 0x3f3f3f3f;
const int mod = 1000000007;
const double PI=3.14159265358979323846264338327950288419716939937510582097494459230;
bool remender(ll a , ll b){return a%b;}
void solve(){
int n;
cin >> n;
ll arr[n];
for(int i = 0; i < n; i++)cin >> arr[i];
ll p[n];
p[n-1] = 0;
for(int i = n-2; i >= 0; i--){
p[i] = p[i + 1];
if(arr[i] <= arr[i + 1])p[i] += (arr[i + 1] - arr[i]) + 1;
}
ll ans = p[0];
ll cur = 0;
for(int i = 1; i < n; i++){
if(arr[i] <= arr[i-1])cur += (arr[i-1] - arr[i]) + 1;
ans = min(ans , max(cur , p[i]));
}
ans = min(ans,cur);
cout << ans << '\n';
}
int main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
//int t;cin >> t;while(t--)
solve();
return 0;
}
