#include <bits/stdc++.h>
using namespace std;
#define pb push_back
#define endl '\n'
#define int long long
const int N=2e5+5,inf=2e18,MOD=1e9+7;
int a[N],pref[N],suff[N];
signed main(){
    ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    int n; cin>>n;
    for(int i=1;i<=n;i++){
        cin>>a[i];
        pref[i]=pref[i-1];
        if(i>1 && a[i]<=a[i-1])pref[i]+=a[i-1]-a[i]+1;
    }
    int sol=inf;
    for(int i=n;i>0;i--){
        suff[i]=suff[i+1];
        if(i<n && a[i+1]>=a[i])suff[i]+=a[i+1]-a[i]+1;
        sol=min(sol,max(pref[i],suff[i]));
        //cout<<pref[i]<<' '<<suff[i]<<' '<<i<<endl;
    }
    cout<<sol;
}
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