#include <bits/stdc++.h>
using namespace std;
//#define int long long
//#define all(x) x.begin(),x.end()
//#define rall(x) x.rbegin(),x.rend()
//#define ff first
//#define ss second
//#define pb push_back
//template<class T, class U> inline bool chmin(T& a, const U& b) { if (a > b) { a = b; return true; } return false; }
//template<class T, class U> inline bool chmax(T& a, const U& b) { if (a < b) { a = b; return true; } return false; }
//mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
//#define rnd(l, r) uniform_int_distribution <int> (l, r)(rng)
const int inf = 1e6, mod = 1e9 + 7;
void solve(){
int n;
cin >> n;
vector<int> a(n);
for(int &i : a)cin >> i;
vector<int> pr(n, 0);
for(int i = 1; i < n; i ++ )pr[i] = pr[i-1] + max(0, a[i-1] - a[i] + 1);
vector<int> sf(n, 0);
for(int i = n - 2; i >= 0; i -- )sf[i] = sf[i+1] + max(0, a[i+1] - a[i] + 1);
int ans = sf[0];
for(int i = 0; i < n; i ++ ){
ans = min(ans, max(sf[i], pr[i]));
}
cout << ans << '\n';
}
signed main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);cout.tie(nullptr);
int tt = 1;
//cin >> tt;
while(tt--){
solve();
};
}
