1355C - Count Triangles - CodeForces Solution

#include<bits/stdc++.h>
using namespace std;
typedef long long LL;
typedef pair<int,int>PII;
const int N=1000010,mod=1e9+7;
LL s[N];
int main()
{
	LL a,b,c,d;
	cin>>a>>b>>c>>d;
	for(int i=a;i<=b;i++)s[i+b]++,s[i+c+1]--;
	for(int i=1;i<N;i++)s[i]+=s[i-1];
	for(int i=N-2;i>=1;i--)s[i]+=s[i+1];
	LL ans=0;
	for(int i=c;i<=d;i++)ans+=s[i+1];
		cout<<ans;
}