#include <bits/stdc++.h>
#define int long long
using namespace std;
const int mx = 2e5+5;
int n;
int a[mx], b[mx], c[mx];
int dp[2][mx];
signed main() {
cin.tie(0);ios::sync_with_stdio(0);
cin >> n;
for (int i=0;i<n;i++) {
cin >> a[i];
}
for (int i=1;i<n;i++) {
dp[0][i] = dp[0][i-1] + max(0LL, a[i-1]+1-a[i]);
}
for (int i=n-2;i>=0;i--) {
dp[1][i] = dp[1][i+1] + max(0LL, a[i+1]+1-a[i]);
}
long long ans = min(dp[0][n-1], dp[1][0]);
for (int i=1;i<n-1;i++) {
long long m = max(dp[0][i-1], dp[1][i+1]);
ans = min(ans, m+max(dp[0][i]-dp[0][i-1],dp[1][i]-dp[1][i+1]));
}
cout << ans << "\n";
}
