#include<iostream>
#include<cstring>
#include <algorithm>
#include<cmath>
using namespace std;
int main(){
	long long n,m,ans=0,flag[33000],s[64000];
	cin>>n>>m;
	for(int i=0;i<n;i++){
		cin>>flag[i];
		s[i]=i+flag[i];
		s[n+i]=flag[i]-i;
	}
	sort(s,s+2*n);
	int j;
	for(int i=0;i<2*n;i++){
		if(i==0||j!=s[i])j=s[i];
		else if(j==s[i])ans++;
	}
	cout<<ans;
    return 0;
}