#include<stdio.h>
#include<stdlib.h>
#include<math.h>
#include<string.h>
#include<stdbool.h>
#include<assert.h>
typedef long long ll;
typedef long double ld;
#define rep(i,l,r)for(ll i=(l);i<(r);i++)
#define repp(i,l,r,k)for(ll i=(l);i<(r);i+=(k))
#define rrep(i,l,r)for(ll i=(l);i>=(r);i--)
#define INF (1LL<<60)
#define MOD1 1000000007
#define MOD2 998244353
#define MAX_N (1 << 17)
#define YES printf("Yes\n")
#define NO printf("No\n")
#define PN printf("\n")
#define charsize 100005 //10^5+5

void swap(ll *a, ll *b){ll c;c=*b;*b=*a;*a= c;}
// void fill(ll n, ll a[n], ll value){ rep(i,0,n) a[i] = value; }
void cin(ll *n){ scanf("%lld",&(*n)); }
void cin2(ll *a, ll *b){ scanf("%lld%lld",&(*a),&(*b)); }
void cin3(ll *a, ll *b, ll *c){ scanf("%lld%lld%lld",&(*a),&(*b),&(*c)); }
ll max2(ll a,ll b){return a>=b?a:b;}
ll min2(ll a,ll b){return a>=b?b:a;}
ll min3(ll a, ll b, ll c){return (a<=b && a<=c) ? a : b<=c ? b : c;}
ll max3(ll a, ll b, ll c){return (a>=b && a>=c) ? a : b>=c ? b : c;}
ll minn(ll n, ll a[n]){ll b=INF;rep(i,0,n) b=min2(b,a[i]);return b;}
ll maxn(ll n, ll a[n]){ll b=-INF;rep(i,0,n) b=max2(b,a[i]);return b;}
ll ABS(ll a){return a>=0?a:(-a);}
ll POW(ll a, ll b){ll c=1;rep(i,0,b) c*=a;return c;}
ll gcd(ll a,ll b){return b?gcd(b,a%b):a;}
ll lcm(ll a,ll b){return a/gcd(a,b)*b;}
ll mod_MOD1(ll n){n+= n<0?((-n)/MOD1+1)*MOD1:0; return n%=MOD1;}
ll mod_p(ll n ,ll p){n+= n<0?((-n)/p+1)*p:0; return n%=p;}
ll change_into_num(char s[] , ll len, ll p){ return !p ? 0 : POW(10,p-1)*(s[len-p]-'0') + change_into_num(s,len,p-1); }

int upll(const void*a, const void*b){return*(ll*)a<*(ll*)b?-1:*(ll*)a>*(ll*)b?1:0;}
int downll(const void*a, const void*b){return*(ll*)a<*(ll*)b?1:*(ll*)a>*(ll*)b?-1:0;}
void sortup(ll*a,int n){qsort(a,n,sizeof(ll),upll);}
void sortdown(ll*a,int n){qsort(a,n,sizeof(ll),downll);}
// int int_sort( const void * a , const void * b ) {
//   return strcmp(( char * )a , ( char * )b );
// }
//     qsort((void*)s,n,sizeof(s[0]),int_sort );

typedef struct{ ll a , b;}fr;
int cmp1( const void *p, const void *q ) { return ((fr*)p) ->a - ((fr*)q)->a;}
int cmp2( const void *p, const void *q ) { return ((fr*)q) ->a - ((fr*)p)->a;}
void strsortup(fr*a,int n){qsort(a,n,sizeof(fr),cmp1);}
void strsortdown(fr*a,int n){qsort(a,n,sizeof(fr),cmp2);}

void lr_lower( int *l, int *r, ll am, ll val , int type ){
    (type<3) ? ( am < val ?  ( *l = (*l+*r)/2 ) : ( *r= (*l+*r)/2 ) ) : ( am <= val ? ( *l = (*l+*r)/2 ) : ( *r= (*l+*r)/2 ) );
    // if(type==1){
    //     am < val ?  ( *l = (*l+*r)/2 ) : ( *r= (*l+*r)/2 );
    // }
    // if(type==2){
    //     am < val ?  ( *l = (*l+*r)/2 ) : ( *r= (*l+*r)/2 );
    // }
    // if(type==3){
    //     am <= val ? ( *l = (*l+*r)/2 ) : ( *r= (*l+*r)/2 );
    // }
    
}
int cmp_lower( ll a, ll b, int type ){
    return (type==1) ? ( a==b ? 1 : 0 ) : (type==2) ? ( a>=b ? 1 : 0 ) : ( a>b ? 1 : 0 ) ;
    // if(type==1) return a==b ? 1 : 0;
    // if(type==2) return a>=b ? 1 : 0;    
    // if(type==3) return a>b ? 1 : 0;
    
    // return 0;
}
/*
 return p  which meets them
 a[p] =  val : type 1  
 a[p] >= val : type 2
 a[p] >  val : type 3
*/
ll lower_bound( ll a[], int l, int r, ll val , int type ){
    while(r-l>1){
        // int m = (l+r)/2;
        lr_lower(&l,&r,a[ (l+r)/2 ],val,type);
        // if(a[m]>val) r=m;
        // else l=m;
    } 
    return cmp_lower(a[l],val,type) ? l : cmp_lower(a[r],val,type) ? r : -1;
}

ll upper_bound( ll a[], int l, int r, ll val ){
    while(r-l>1){
        int m = (l+r)/2;
        if(a[m]>val) r=m;
        else l=m;
    } 
    return a[r]==val ? r : a[l]==val ? l : -1;
}


/*ai(i=l~r)のうち、ai=xとなるiの個数*/
ll count(ll n, ll a[], ll l, ll r, ll x){
    ll p = lower_bound(a,l,r,x,1);
    return p==-1 ? 0 : upper_bound(a,p,r,x)-p+1;
}

int main(void){
    // char ;
    ll n;
    ll ans=0;
    cin(&n);
    // cin(&);
    // cin(&);
    // cin2(&,&);
    // cin3(&,&,&);
    // scanf("%s",);
    ll a[n],sub[n-1],used[n],mark=0,sub2[n-1];
    rep(i,0,n) used[i] = 0;
    rep(i,0,n){
        cin(&a[i]);
        if(i){
            sub[i-1] = a[i]-(i+1);
        }
        if(i!=n-1){
            sub2[i] = -a[i]-(i+1);
        }
    }
    sortup(sub,n-1);
    sortup(sub2,n-1);

    // rep(i,0,n-1) printf("%lld ",sub[i]);
    // PN;
    // rep(i,0,n-1) printf("%lld ",sub2[i]);
    // PN;
    // rep(i,0,n-1){
    //     ll val= -a[i]-(i+1),judge=0;
    //     rep(j,0,mark){
    //         if(val == used[j]){
    //             judge++;
    //         }
    //     }
    //     printf("%lld ",val);
    //     printf("%lld %lld ",lower_bound4(n-1,sub,0,n-2,val) , upper_bound(n-1,sub,0,n-2,val)  );
    //     ans+=count(n-1,sub,0,n-2,val);
    //     // printf("%lld ",ans);
    //     ans-=judge;
    //     used[mark] = a[i]-(i+1);
    //     printf("%lld \n",ans);
    //     mark++;
    // }

    rep(i,0,n-1){
        ll p = count(n-1,sub2,0,n-2, sub[i] );
        // printf("%lld ",p);
        ans+=p;
    }
    
    printf("%lld\n",ans);
    return 0;
}


Measured memory_bytes: 9552