#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
#include "debug.h"
#else
#define debug(...) 42
#endif
const int N = 2e5 + 5;
const long long inf = 1e18;
int n;
int a[N], d[N];
long long pf[N], sf[N];
int main() {
ios::sync_with_stdio(false); cin.tie(nullptr);
cin >> n;
for (int i = 1; i <= n; ++i) {
cin >> a[i];
}
for (int i = 1; i < n; ++i) {
d[i] = a[i] - a[i + 1];
}
for (int i = 1; i < n; ++i) {
pf[i] = pf[i - 1] + max(0, d[i] + 1);
}
for (int i = n - 1; i >= 1; --i) {
sf[i] = sf[i + 1] + max(0, -d[i] + 1);
}
long long res = inf;
for (int i = 1; i <= n; ++i) {
res = min(res, pf[i - 1] + sf[i]);
}
cout << res;
return 0;
}
