#include "bits/stdc++.h"
using namespace std;
const int maxn = 2e5 + 5;
int n, a[maxn];
long long f[maxn], g[maxn];
long long pref[maxn], suf[maxn];
long long b[maxn], c[maxn];
void solve() {
cin >> n;
for(int i = 1; i <= n; ++i) {
cin >> a[i];
}
for(int i = 1; i <= n; ++i) {
pref[i] = max(pref[i - 1] + 1, 1ll * a[i]);
b[i] = max(0ll, pref[i] - a[i]);
f[i] = f[i - 1] + max(0ll, b[i] - b[i - 1]);
}
for(int i = n; i; --i) {
suf[i] = max(suf[i + 1] + 1, 1ll * a[i]);
c[i] = max(0ll, suf[i] - a[i]);
g[i] = g[i + 1] + max(0ll, c[i] - c[i + 1]);
}
long long ans = min(f[n], g[1]);
for(int i = 1; i < n; ++i) {
ans = min(ans, max(f[i], g[i]));
}
cout << ans;
}
signed main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
solve();
return 0;
}
