#include <bits/stdc++.h>
using namespace std;
#define int long long
#define fi first
#define se second
#define sz(a) (int)a.size()
#define all(a) a.begin(),a.end()
#define pb push_back
const int mxN = (int)2e5+10;
int n, a[mxN], pre[mxN], suf[mxN];
void solve(){
cin >> n;
for(int i = 1; i <= n; i++) cin >> a[i];
for(int i = 2; i <= n; i++)
pre[i] = pre[i-1]+max(0ll, a[i]-a[i-1]+1);
for(int i = n-1; i >= 1; i--)
suf[i] = suf[i+1]+max(0ll, a[i]-a[i+1]+1);
int ans = min(suf[1], pre[n]);
for(int i = 1; i < n; i++) ans = min(ans, pre[i]+suf[i+1]+(a[i]+pre[i]==a[i+1]+suf[i+1]));
cout << ans;
}
int32_t main() {
ios_base::sync_with_stdio(false); cin.tie(0);
int t = 1; //cin >> t;
while(t--) solve();
}
