#include <float.h>
#include <inttypes.h>
#include <limits.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <unistd.h>
#include <math.h>
#ifdef __cplusplus
#include <bits/stdc++.h>
#endif

uint32_t nextpint(void){ char c=getchar_unlocked(); while(c<'0'||'9'<c) c=getchar_unlocked(); uint32_t x=0; while('0'<=c && c<='9'){ x=x*10+c-'0'; c=getchar_unlocked(); } return x; }
void printint(uint32_t x){ if(x<0){x=-x; putchar_unlocked('-');}else if(x==0){putchar_unlocked('0');} int d=0; char s[10]; while(x){ s[d]='0'+x%10; x/=10; d++; } for (int i=d-1; i>=0; i--) putchar_unlocked(s[i]); putchar_unlocked('\n'); }
void rsort_d32(void *base, const int count);

int32_t a[200000];
int32_t b[200000];

int main(void){
	uint32_t n=nextpint();
	for(uint32_t i=0; i<n; i++){
		int32_t k=nextpint();
		a[i]=i+k;
		b[i]=i-k;
	}
	rsort_d32(a, n);
	rsort_d32(b, n);
	uint64_t ans=0;
	uint32_t al=0, bl=0;
	while(al<n){
		int32_t A=a[al];
		while(bl<n && b[bl]<A) bl++;
		if(bl==n) break;
		if(b[bl]==A){
			int32_t ac=0, bc=0;
			while(al<n && a[al]==A) al++, ac++;
			while(bl<n && b[bl]==A) bl++, bc++;
			ans+=(int64_t)ac*bc;
		}else{
			al++;
		}
	}
	printf("%"PRIu64"\n", ans);
}

void rsort_d32(void *base, const int count){
	int32_t *a=base;
	int32_t *b=malloc(count*sizeof(int32_t));
	int c[0x100];
	memset(c, 0, sizeof(c)); for(int i=0; i<count; i++){ c[(a[i]>>(0<<3))&0xFF]++; } for(int i=1; i<0x100; i++) c[i]+=c[i-1]; for(int i=count-1; i>=0; i--){ b[--c[(a[i]>>(0<<3))&0xFF]]=a[i]; }{void *t=a; a=b; b=t;}
	memset(c, 0, sizeof(c)); for(int i=0; i<count; i++){ c[(a[i]>>(1<<3))&0xFF]++; } for(int i=1; i<0x100; i++) c[i]+=c[i-1]; for(int i=count-1; i>=0; i--){ b[--c[(a[i]>>(1<<3))&0xFF]]=a[i]; }{void *t=a; a=b; b=t;}
	memset(c, 0, sizeof(c)); for(int i=0; i<count; i++){ c[(a[i]>>(2<<3))&0xFF]++; } for(int i=1; i<0x100; i++) c[i]+=c[i-1]; for(int i=count-1; i>=0; i--){ b[--c[(a[i]>>(2<<3))&0xFF]]=a[i]; }{void *t=a; a=b; b=t;}
	memset(c, 0, sizeof(c)); for(int i=0; i<count; i++){ c[(a[i]>>(3<<3))+0x80]++; } for(int i=1; i<0x100; i++) c[i]+=c[i-1]; for(int i=count-1; i>=0; i--){ b[--c[(a[i]>>(3<<3))+0x80]]=a[i]; }{void *t=a; a=b; b=t;}
	free(b);
}
