#include <bits/stdc++.h>
#define ll long long
#define fi first
#define se second
#define sz(x) (int)x.size()
#define ALL(v) v.begin(),v.end()
#define MASK(k) (1LL << (k))
#define BIT(x, i) (((x) >> (i)) & 1)
#define oo (ll)1e18
#define INF (ll)1e9
#define MOD (ll)(1e9 + 7)
using namespace std;
template<class T1, class T2>
bool maximize(T1 &a, T2 b){if(a < b){a = b; return true;} return false;}
template<class T1, class T2>
bool minimize(T1 &a, T2 b){if(a > b){a = b; return true;} return false;}
template<class T1, class T2>
void add(T1 &a, T2 b){a += b; if(a >= MOD) a -= MOD;}
template<class T1, class T2>
void sub(T1 &a, T2 b){a -= b; if(a < 0) a += MOD;}
template<class T>
void cps(T &v){sort(ALL(v)); v.resize(unique(ALL(v)) - v.begin());}
const int MAX = 2e5 + 10;
int N;
int a[MAX];
void solve(){
cin >> N;
for(int i = 1; i <= N; i++){
cin >> a[i];
}
ll L = 0, R = 0;
for(int i = 1; i < N; i++){
if(a[i] <= a[i + 1]) R += a[i + 1] - a[i] + 1;
}
ll ans = oo;
for(int i = 1; i <= N; i++){
minimize(ans, max(L, R));
// cout << i << ' ' << L << ' ' << R << '\n';
if(a[i] >= a[i + 1]) L += a[i] - a[i + 1] + 1;
if(a[i] <= a[i + 1]) R -= a[i + 1] - a[i] + 1;
}
cout << ans << '\n';
}
int main(){
ios_base::sync_with_stdio(0); cin.tie(0);
// freopen("CAU3.inp","r",stdin);
// freopen("CAU3.out","w",stdout);
int t = 1;
// cin >> t;
while(t--){
solve();
}
return 0;
}
