#include <bits/stdc++.h>
using namespace std;
#define int long long
//#define all(x) x.begin(),x.end()
//#define rall(x) x.rbegin(),x.rend()
//#define ff first
//#define ss second
//#define pb push_back
//template<class T, class U> inline bool chmin(T& a, const U& b) { if (a > b) { a = b; return true; } return false; }
//template<class T, class U> inline bool chmax(T& a, const U& b) { if (a < b) { a = b; return true; } return false; }
//mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
//#define rnd(l, r) uniform_int_distribution <int> (l, r)(rng)
const int inf = 1e6, mod = 1e9 + 7;
void solve(){
int n;
cin >> n;
vector<int> a(n);
for(int &i : a)cin >> i;
if(n == 1){
cout << 0 << '\n';
return;
}
int l = 1, r = n-2, ans = 0;
while(1){
if(l == r+1 && a[l] == a[r]){ans ++ ;break;}
if(l > r)break;
int lf = a[l-1] - a[l] + 1;
int ri = a[r+1] - a[r] + 1;
if(lf <= 0){l ++; continue;}
if(ri <= 0){r --; continue;}
if(lf < ri){
a[l] += lf;
a[r] += lf;
ans += lf;
}else{
a[l] += ri;
a[r] += ri;
ans += ri;
};
};
cout << ans << '\n';
}
signed main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);cout.tie(nullptr);
int tt = 1;
//cin >> tt;
while(tt--){
solve();
};
}
