#include<bits/stdc++.h>
using namespace std;
int main(){
  int n,m;cin>>n>>m;
  vector<int> a(m),t(m,1);
  for(int i=0;i<m;i++){
    cin>>a[i];
    if(a[i]>i)t[i]=a[i]-i;
    else t[i]=t[i-a[i]];
  }
  cout<<t[m-1]<<endl;
}