#include <bits/stdc++.h>
using namespace std;
#define int long long
#define N 500005
const int inf = 1e18;
int n, a[N], l[N], r[N], ans = inf, pf[N], sf[N], b[N];
int32_t main() {
ios::sync_with_stdio(0);cin.tie(0);
cin >> n;
for(int i = 1; i <= n; i++) {
cin >> a[i];
b[i] = a[i];
}
for(int i = 1; i <= n; i++) {
l[i] = l[i - 1] + max(0ll, a[i - 1] - a[i] + 1);
}
int op = 0, mx = 0;
for(int i = n; i >= 1; i--) {
r[i] = r[i + 1] + max(0ll, a[i + 1] - a[i] + 1);
}
for(int i = 1; i <= n; i++) {
ans = min(ans,max(l[i], r[i]));
}
cout << ans << endl;
return 0;
}
