#include <bits/stdc++.h>
#define int long long
using namespace std;
int tab[200005];
int R[200005];
int pref[200005];
int suf[200005];
int32_t main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
int n;
cin >> n;
for(int i = 1; i <= n; i++){
cin >> tab[i];
R[i] = tab[i] - tab[i-1];
}
for(int i = 2; i <= n; i++){
pref[i] = pref[i-1];
if(R[i] <= 0) pref[i] += (-R[i])+1;
}
for(int i = n-1; i > 0; i--){
suf[i] = suf[i+1];
if(R[i+1] >= 0) suf[i] += R[i+1]+1;
}
int res = 1e18;
for(int i = 1; i <= n; i++){
res = min(res, max(suf[i], pref[i]));
}
cout << res << '\n';
return 0;
}
