#include <bits/stdc++.h>
using namespace std;
#define fi first
#define se second
typedef long long ll;
typedef pair<int,int> ii;
const int N = 2e5;
const ll INF = 1e18;
int n;
ll a[N+5], pref[N+2], suff[N+2];
int main() {
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
cin >> n;
for (int i = 1; i <= n; i++) {
cin >> a[i];
}
for (int i = 2; i <= n; i++) {
pref[i] = pref[i-1]+max(0ll,a[i-1]-a[i]+1);
}
for (int i = n-1; i >= 1; i--) {
suff[i] = suff[i+1]+max(0ll,a[i+1]-a[i]+1);
}
ll ans = INF;
for (int i = 1; i <= n; i++) {
// ans = min(ans,max(pref[i],suff[i]));
ans = min(ans,pref[i]+suff[i]);
}
cout << ans << '\n';
return 0;
}
