#include<bits/stdc++.h>
using namespace std;
using ll = long long;
int main() {
ios::sync_with_stdio(0);
cin.tie(0);
int n;
cin >> n;
int a[n];
ll b[n - 1];
for(int i = 0; i < n; i++) {
cin >> a[i];
}
for(int i = 0; i < n - 1; i++) {
b[i] = a[i] - a[i + 1];
}
ll c[n] {}, d[n] {};
for(int i = 0; i < n - 1; i++) {
c[i + 1] = c[i] + max(0LL, 1 + b[i]);
}
for(int i = n - 2; i >= 0; i--) {
d[i] = d[i + 1] + max(0LL, 1 - b[i]);
}
ll ans = 1e18;
for(int i = 0; i < n; i++) {
ans = min(ans, max(c[i], d[i]));
}
cout << ans << '\n';
return 0;
}
