Codeforces Round #266 (Div. 2), problem: (C) Number of Ways

#pragma GCC optimize("Ofast")
#pragma GCC target("avx,avx2,fma")
#pragma GCC optimization ("unroll-loops")

#include<bits/stdc++.h>
using namespace std;
#define pb push_back
#define pii pair<int,int>
#define pli pair<long long int,int>
#define pbi pair<bool,int>
#define pob pop_back
#define all(a) a.begin(),a.end()
#define fio ios_base::sync_with_stdio(false); cin.tie(NULL);cout.tie(NULL);
#define i64 long long int
#define mem(x,y) memset(x,y,sizeof(x))
#define fill(arr,b) fill(arr, arr+sizeof(arr)/sizeof(arr[0]), b)
#define maxarr(n,arr,m) m=-1e9;for(int i=0;i<n;i++)if(arr[i]>m) m=arr[i];
#define read freopen("input.txt","r",stdin);
#define F first
#define S second

const int maxn = (i64)1e9+7;
const double pi = acos(-1.0);

//bool fne(const pii &a, const pii &b){return (a.F < b.F)||(a.F==b.F&&a.S<b.S);}
//bool sne(const pii &a, const pii &b){return (a.S < b.S)||(a.S==b.S&&a.F<b.F);}

int arr[500005];

int main()
{
    int n;
    i64 sum=0;
    scanf("%d",&n);
    for(int i=0;i<n;i++) {scanf("%d",&arr[i]);sum+=arr[i];}
    if(n<3||sum%3!=0) {return printf("0"),0;}
    i64 s=0,ans=0,cnt=0;
    sum/=3;
    if(sum==0)
    {
        for(int i=0;i<n-1;i++){s+=arr[i];if(s==0) ans++;}
        return printf("%lld",(ans*(ans-1))>>1),0;
    }
    for(int i=0;i<n-1;i++)
    {
        s+=arr[i];
        if(s==sum) cnt++;
        else if(s==sum*2)ans+=cnt;
    }
    printf("%lld",ans);

    return 0;
}