<span>Codeforces 1355 C. Count Triangles</span>

 1 #include<bits/stdc++.h>
 2 #define ll long long
 3 using namespace std;
 4 
 5 int main()
 6 {
 7     ios::sync_with_stdio(false);
 8     cin.tie(0);
 9     cout.tie(0);
10     ll a,b,c,d;
11     cin>>a>>b>>c>>d;
12     ll ans=0;
13     for(ll i=a;i<=b;i++){
14         ll p=i+b-1;
15         ll q=i+c-1;
16         ll x=0,y=0,pp=0,qq=0;
17         if(p>=c) x=p-c+1;
18         if(q>=c) y=q-c+1;
19         if(p>d) pp=p-d;
20         if(q>d) qq=q-d;
21         ans=ans+(x+y)*abs(y-x+1)/2;
22         ans=ans-(pp+qq)*abs(qq-pp+1)/2;
23     }
24     cout<<ans<<endl;
25     return 0;
26 }