#include <bits/stdc++.h>
using namespace std;
#define pb push_back
#define mp make_pair
typedef long long ll;
typedef pair<long long, long long> pll;
typedef pair<int,int> pii;
typedef vector<long long> vl;
typedef vector<int> vi;
int main(){
int n;
cin>>n;
vi arr(n);
for(int i = 0; i < n; i++) cin>>arr[i];
vector<ll> pref(n), suf(n);
for(int i = 0; i < n-1; i++){
pref[i+1] = pref[i] + max(0, arr[i] - arr[i+1] + 1);
}
for(int i = n-2; i >= 0; i--){
suf[i] = suf[i+1] + max(0, arr[i+1] - arr[i] + 1);
}
ll ans = 2e15;
for(int i = 0; i < n; i++){
ans = min(ans, max(pref[i], suf[i]));
}
cout<<ans<<endl;
return 0;
}
