#include<bits/stdc++.h>
using namespace std;

int main(){
    int n,in,sum=0,ans=0;
    vector<int> v;
    cin >> n;
    while(cin >> in){
        v.push_back(in);sum+=in;
    }
    int tmp=0,l=0,top;
    for(int i=1;;i+=2){
        tmp+=i;
        if(tmp>sum) break;
        if(i==1) l++;
        else l+=2;
        top=i/2+1;
    }
    int p=(l>n)?l:n;
    for(int i=0;i<p;i++){
        if(i>=n) continue;
        if(top>i){
            if(i+1<v[i]) ans+=v[i]-(i+1);
        }else{
            int b=(top-(i+1-(l/2+1))>0)?top-(i+1-(l/2+1)):0;
            if(b<v[i]) ans+=v[i]-b;
        }
    }
    cout << ans << endl;

    return 0;
}