#include <bits/stdc++.h>
#define ll long long
#define ull unsigned ll
#define pb push_back
#define pa pair<int, int>
#define pall pair<ll, int>
#define fi first
#define se second
#define TASK "test"
#define Size(x) (int) x.size()
#define all(x) x.begin(), x.end()
using namespace std;
template<typename T1, typename T2> bool mini (T1 &a, T2 b) {if(a > b) a = b; else return 0; return 1;}
template<typename T1, typename T2> bool maxi (T1 &a, T2 b) {if(a < b) a = b; else return 0; return 1;}
const int MOD = 1e9 + 7;
const int LOG = 20;
const int maxn = 2e5 + 7;
const ll oo = 1e18 + 69;
int n, a[maxn];
ll pre[maxn], suf[maxn];
int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
//freopen(TASK".inp", "r", stdin);
//freopen(TASK".out", "w", stdout);
cin>>n;
for(int i = 1; i <= n; i++) {
cin>>a[i];
if(i > 1) pre[i] = pre[i - 1] + max(0, a[i - 1] + 1 - a[i]);
}
ll ans = oo;
for(int i = n; i >= 1; i--) {
if(i < n) suf[i] = suf[i + 1] + max(0, a[i + 1] + 1 - a[i]);
mini(ans, max(pre[i], suf[i]));
}
cout<<ans;
return 0;
}
