#include <bits/stdc++.h> using namespace std; #define endl '\n' #define dl double #define int long long #define ul uint_fast32_t #define ll int_fast64_t #define dll long double #define ull uint_fast64_t #define spektar this_thread::sleep_for(chrono::milliseconds(50)) void solve(){ int n; cin >> n; vector<ll> k(n); for(auto& i:k) cin >> i; vector<ll> pref1(n),pref2(n); pref1[0]=0; pref2[n-1]=0; for(int i=1; i<n; i++){ if(k[i-1]+1>k[i]) pref1[i]=pref1[i-1]+k[i-1]+1-k[i]; else pref1[i]=pref1[i-1]; } for(int i=n-2; i>=0; i--){ if(k[i+1]+1>k[i]) pref2[i]=pref2[i+1]+k[i+1]+1-k[i]; else pref2[i]=pref2[i+1]; } ll min1=1000000000000000000; for(int i=0; i<n; i++) min1=min(min1,max(pref2[i],pref1[i])); cout << min1 << endl; } signed main(){ //freopen("input.txt","r",stdin); //freopen("output.txt","w",stdout); ios_base::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr); int t=1; //cin >> t; while(t--){ solve(); } }