#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
#define rep(a, b) for(int a = 0; a < (b); ++a)
#define st first
#define nd second
#define pb push_back
#define all(a) a.begin(), a.end()
const int LIM=2e5+7, INF=1e9+7;
int T[LIM], pref[LIM], suf[LIM];
int main() {
ios_base::sync_with_stdio(0); cin.tie(0);
int n;
cin >> n;
rep(i, n) cin >> T[i];
for(int i=1; i<n; ++i) {
pref[i]=pref[i-1];
if(T[i-1]>=T[i]) pref[i]+=T[i-1]-T[i]+1;
}
for(int i=n-2; i>=0; --i) {
suf[i]=suf[i+1];
if(T[i+1]>=T[i]) suf[i]+=T[i+1]-T[i]+1;
}
int ans=INF;
rep(i, n) ans=min(ans, max(pref[i], suf[i]));
cout << ans << '\n';
}
