#include <bits/stdc++.h>
#define int long long
#define pp pair <bool,int>
#define ppi pair <int,int>
#define tp tuple <int,int,int,bool>
#define c1(x) cin >> x
#define c2(x,y) cin >> x >> y
#define c3(x,y,z) cin >> x >> y >> z
#define c4(x,y,z,f) cin>> x >> y >> z >> f
#define mp(a,b) make_pair(a,b)
using namespace std;
void solve(){
int n;cin>>n;
unordered_map<int,bool> mp;
for (int i=0;i<n;++i){
int a;cin>>a;
mp[a+i] = true;
}
cout << mp.size();
}
int32_t main(){
ios_base::sync_with_stdio(0);cin.tie(0);
solve();
}
// 1 6 9 3 4 10 3 9
