codeforces 153 A. Points on Line 简单数学题 简单推导推导的事情

#include<iostream>
#include<cstdio>
#include<cstring>
using namespace std;
const int maxn=100002;
int n,d,a[maxn],q[maxn];
long long int  ans;
int main()
{
while(~scanf("%d%d",&n,&d))
{
int i,j,l=1,r=1;
ans=0;
for(i=1;i<=n;i++)
{
scanf("%d",&a[i]);
while(l<r&&a[i]-a[l]>d) l++;
ans+=(long long)(r-l)*(r-l-1)/2;
++r;
}
cout<<ans<<endl;
}
return 0;
}
/*
4 3
1 2 3 4

4 2
-3 -2 -1 0
*/