#include <cstdio>
#include <cstdlib>
#include <iostream>
#include <string>
#include <cmath>
#include <algorithm>
#include <vector>
#include <queue>
#include <stack>
#include <map>

using namespace std;

int main(){
    int num[101],n,ans=0;
    
    scanf("%d",&n);
    
    for(int i=1;i<=n;i++){
        scanf("%d",&num[i]);
        if(i<=2/n+(n%2))
            ans+=max(num[i]-i,0);
        else
            ans+=max(num[i]-(n-i+1),0);
    }

    printf("%d\n",ans);
    
    return 0;
}