/// #if (code == true)
#include <bits/stdc++.h>
using namespace std;
#define pll pair<long long, long long>
#define len(a) ((int)((a).size()))
#define all(a) a.begin(), a.end()
#define add push_back
#define mkp make_pair
#define ll long long
#define fr first
#define sc second
const long long INF = 1000000000ll * 1000000003ll;
const long long MOD = 1000000007ll;
const int N = 2e5 + 5;
ll n, a[N], pref[N], suf[N];
void solve(){
cin >> n;
for(int i = 0; i < n; i++){
cin >> a[i];
if(i > 0) pref[i] = pref[i - 1] + max(0ll, a[i - 1] + 1 - a[i]);
}
for(int i = n - 2; i >= 0; i--){
suf[i] = suf[i + 1] + max(0ll, a[i + 1] + 1 - a[i]);
}
ll ans = min(suf[0], pref[n - 1]);
for(int i = 1; i < n - 1; i++){
ans = min(ans, max(suf[i], pref[i]));
// if(ans == 11){
// cout << i << " " << pref[i] << " " << suf[i] << endl;
// }
}
cout << ans << "\n";
}
int main(){
ios_base::sync_with_stdio(false);
cin.tie(0);
int _ = 1;
// cout << fixed;
// cout.precision(15);
// cin >> _ ;
while(_--) solve();
return 0;
}
/// #else
/// #include <bits/stdc++.h> using namespace std; int main() { cout << "Hello World!"; }
/// #endif
