// C++(Clang 10.0.0)

//GCC用 ナゾです
//#pragma GCC target("avx")
//#pragma GCC optimize("O3")
//#pragma GCC optimize("unroll-loops")

#include <iostream>
#include <stdio.h>
#include <stdlib.h> //rand() 0~RAND_MAX2147483647
#include <math.h>
#include <time.h>   //srand((unsigned int)time(NULL));
#include <unistd.h> //sleep
#include <algorithm>
#include <cmath>
#include <vector>
#include <regex> //正規表現
#include <map>
#include <unordered_map>
#include <set>
#include <cassert> //assart(x>=0) -> x<0の時異常終了
#include <numeric>
#include <iterator>
#include <list>
#include <tuple>
#include <queue>

using namespace std;
#define ll long long int
#define FOR(i,min,max) for(ll i=(min);i<(max);i++)
#define FOReq(i,min,max) for(ll i=(min);i<=(max);i++)
#define FORREV(i,max,min) for(ll i=(max)-1;i>=(min);i--)
#define FORREVeq(i,max,min) for(ll i=(max);i>=(min);i--)

#define all(vec) (vec).begin(),(vec).end()

const int MOD=1e9+7;
const int MOD2=998244353;
const int not0=1;
const int INF=2000000001;
const ll INFll=2e18;
//const int MAX_V=10000;
const double  Micro=1e-5;

int UnUsedVariable(){
    if(INF || Micro || MOD || INFll || MOD2 || not0){;}
    return 0;
}

#define inarray(array,min,max) FOR(i,min,max){cin>>array[i];}
#define inarrayW(arrayA,arrayB,min,max) FOR(i,min,max){cin>>arrayA[i]>>arrayB[i];}
//#define br printf("\n");
#define br cout<<"\n";
#define OutputAndReturn0(x) {cout<<(x)<<endl;return 0;}
#define elif else if

#define issame(a,b,c,d) ((a)==(b) and (b)==(c) and (c)==(d))
#define isdiff3(a,b,c) ((a)!=(b) and (b)!=(c) and (c)!=(a))
#define isdiff4(a,b,c,d) ((a)!=(b) and (b)!=(c) and (c)!=(a) and (d)!=(a) and (d)!=(b) and (d)!=(c))
#define isdiff5(a,b,c,d,e) ((a)!=(b) and (b)!=(c) and (c)!=(a) and (d)!=(a) and (d)!=(b) and (d)!=(c) and (e)!=(a) and (e)!=(b) and (e)!=(c) and (e)!=(d))
#define Func3(f,a,b,c) (f)((f)((a),(b)),(c))
#define Func4(f,a,b,c,d) (f)((f)((f)((a),(b)),(c)),(d))

inline int isEven(int x){return x%2==0;}
inline int isOdd (int x){return x%2==1;}
inline ll samepair(ll x){return x*(x-1)/2;}
inline void printD(double d){printf("%15.15lf ",d);}
inline ll  gcd(ll a,ll b){ if(b==0)return a;else return gcd(b,a%b);}//最大公約数 なお一方が0なら答えは他方になるので番兵は０で良い また、負の数もOK -4%3=-1となるため
inline ll  lcm(ll a,ll b){return a*b/gcd(a,b);}//MODやオーバーフローには注意！
//inline ll  ceil(double a){return (ll)a+1;}
//ans = accumulate(A, A+N, 0, [](int a, int b) {return gcd(a,b);});
//auto count = std::count_if(v.begin(), v.end(), [](int x) { return x == 1 || x == 3; } );

ll dx[4]={1,0,-1,0};
ll dy[4]={0,1,0,-1};

//番兵　累積和　しゃくとり法　木構造　DP キュー　乱択アルゴリズム　嘘解法　ハッシュ　next_permulation

//課題：DP 木構造　Xor

ll probA(){
    ll ans;ans=0;
    string K;
    cin>>K;
    
    if(K=="ABC"){OutputAndReturn0("ARC")}else OutputAndReturn0("ABC")
    
    
    OutputAndReturn0(ans);
    return 0;
}

ll probB(){
    ll ans;ans=0;
    ll N,K;N=0;
    cin>>N>>K;
    
    ll okasi[N];
    FOR(i,0,N){
        okasi[i]=0;
    }
    
    ll d,tmp;
    FOR(i, 0, K){
        cin>>d;
        FOR(i, 0, d){
            cin>>tmp;tmp--;
            okasi[tmp]++;
        }
    }
    
    FOR(i, 0, N){
        if(okasi[i]==0)ans++;
    }
    
    
    
    OutputAndReturn0(ans);
    return 0;
}

ll probC(){
    ll ans;ans=0;
    
    
    int N,M;cin>>N>>M;
    
    ll H[N];
    inarray(H, 0, N);
    
    vector<vector<int>>graph(N);
    for(int i=0;i<M;++i){
        int a,b;cin>>a>>b;a--;b--;
        graph[a].push_back(b);
        graph[b].push_back(a);
    }
    
    const ll top=1;
    const ll nottop=-1;
    const ll undef=0;
    
    ll OK[N];
    FOR(i, 0, N){
        OK[i]=undef;
    }
    
    bool flag;
    FOR(i, 0, N){
        if(OK[i]==nottop)continue;
        flag=true;
        
        
        for (auto& e:graph[i]) {
                 if(H[i] <H[e]){flag=false;break;}
            else if(H[i]==H[e]){flag=false;OK[e]=nottop;break;}
            else if(H[i] >H[e]){continue;}
        }
        
        
        if(flag==true){
            OK[i]=top;
            for (auto& e:graph[i]) {
                OK[e]=nottop;
            }
        }
        else OK[i]=nottop;
        
    }
    
    FOR(i, 0, N){
        if(OK[i]==top)ans++;
    }

    
    OutputAndReturn0(ans);
    return 0;
}

ll probD(){
    ll ans;ans=0;
    ll A,B,N;N=0;
    cin>>A>>B>>N;
  
    OutputAndReturn0( ans  );
    return 0;
}

ll tosmall[200010]={0};
ll tobig[200010]={0};


ll probE(){
    ll ans;ans=0;
    ll N;N=0;
    cin>>N;
    
    ll A[N+1];
    A[0]=INF;
    inarray(A, 1, N+1);
    
    FOReq(i, 1, N){
        if(0<i-A[i] and i-A[i]<200010){tosmall[i-A[i]]++;}
        if(0<i+A[i] and i+A[i]<200010){tobig[i+A[i]]++;}
    }
    
    FOReq(i, 1, N){
        ans+= tobig[i]*tosmall[i];
    }
   
    OutputAndReturn0(ans);
    return 0;
}

ll probF(){
    ll ans;ans=0;
    ll N;N=0;
    cin>>N;
    OutputAndReturn0(ans);
    return 0;
}

signed main(){
    cin.tie(0);    //cinとcoutの結び付き解除（cout時にcinの内容を吐き出さなくなる）
    ios::sync_with_stdio(false);  //coutとprintfの結び付き解除（coutとprintfを同時に使うな！）
    probE();
    return 0;
}


Measured memory_bytes: 7860