#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define pb push_back
#define fi first
#define se second
constexpr ll MAXN=5e5+5,MAXV=1e5,MOD=1e9+7,INF=1e18;
ll n,m,i,j,p,k,ans,dem,st,en,a[MAXN],dp1[MAXN],dp2[MAXN],b[MAXN],last1[MAXN],last2[MAXN];
int main(){
ios::sync_with_stdio(0);
cin.tie(0);cout.tie(0);
// freopen("test.inp","r",stdin);
// freopen("test.out","w",stdout);
cin>>n;
for(i=1;i<=n;i++) cin>>a[i];
ll l=1,r=n,ans=0,need1=0,needr=0;
while(l<=r){
if(a[l]+need1>a[l-1]){
a[l]+=need1;
l++;
}
else if(a[r]+needr>a[r+1]){
a[r]+=needr;
r--;
}
else{
ll nl=a[l-1]-(a[l]+need1)+1,nr=a[r+1]-(a[r]+needr)+1;
if(nl<=nr){
need1+=nl;
needr+=nl;
a[l]+=need1;
l++;
}
else{
need1+=nr;
needr+=nr;
a[r]+=needr;
r--;
}
}
}
if(a[r]==a[l]) need1++;
cout<<need1;
}
