#include <bits/stdc++.h>
typedef long long ll;
using namespace std;
#define f first
#define s second
#define pb push_back
#define ep emplace
#define eb emplace_back
#define lb lower_bound
#define ub upper_bound
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define uniquev(v) sort(all(v)), (v).resize(unique(all(v)) - (v).begin())
#define mem(f,x) memset(f , x , sizeof(f))
#define sz(x) (int)(x).size()
#define __lcm(a, b) (1ll * ((a) / __gcd((a), (b))) * (b))
#define mxx *max_element
#define mnn *min_element
#define cntbit(x) __builtin_popcountll(x)
#define len(x) (int)(x.length())
const int N = 5e5 + 100;
int a[N], f[N];
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int n;
cin >> n;
for (int i = 1; i <= n; i++) {
cin >> a[i];
f[i] = a[i] - a[i - 1];
}
ll l = 0, r = 0;
for (int i = 2; i <= n; i++) {
if (f[i] >= 0) {
r += f[i] + 1;
}
}
ll ans = r;
for (int i = 2; i <= n; i++) {
if (f[i] >= 0) {
r -= (f[i] + 1);
}
if (f[i] <= 0) {
l += (abs(f[i]) + 1);
}
ans = min(ans, l + r - min(l, r));
}
cout << ans << '\n';
return 0;
}
// a[l] += 1
// a[r + 1] -=
