#include<iostream>
using namespace std;

int main(){
    int N,bar=1,ans=0,tmp;
    cin >> N;
    int* blocks = new int[N];

    for(int i=0;i<N;i++){
        cin >> blocks[i];
    }

    for(int i=0;i<N;i++){
        tmp = blocks[i]-bar;
        if(tmp==0){
            bar+=1;
        }else if(tmp>0){
            ans+=tmp;
            bar=blocks[i]+1;
        }
    }
    cout << ans << endl;
    return 0;
}