Language: C

#include <iostream>
#include <algorithm>
#include <string>
#include <vector>
#include <set>
#include <cmath>
#include <queue>
#include <map>
#include <stack>
#include <bitset>
#include <numeric> //lcm
#include <iomanip> //double精度 setprecision

using namespace std;

#define rep(i,n) for(int i = 0; i < (n); ++i)
#define rrep(i,n) for(int i = n-1; i >= 0; --i)
#define rep1(i,n) for(int i = 1; i <= (n); ++i)
#define rrep1(i,n) for(int i = (n); i >= 1; --i)
#define REP(i,n,m) for(int i = (n); i < (m); ++i)
#define all(vec) (vec).begin(),(vec).end()

#define debug(vec) for(auto v : vec) cerr << v << " "; cerr << endl;
#define debug2D(vec2D) for(auto vec : vec2D) { for (auto v : vec) cerr << v << " "; cerr << endl; } 
#define debugP(vec) for(auto v : vec) cerr << "(" << v.first << "," << v.second << ") "; cerr << endl;
#define debug2DP(vec2D) for(auto vec : vec2D) { for (auto v : vec) cerr << "(" << v.first << "," << v.second << ") "; cerr << endl; } 

typedef long long ll;

constexpr ll INF = 1000000000; //10^10
//const ll MOD = 998244353;
constexpr ll MOD = 1000000007;

template<class T>inline bool chmax(T& a, T b) { if (a <= b) { a = b; return true; } else return false; }
template<class T>inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } else return false; }

ll gcd(ll a, ll b) {
  if(a < b) return gcd(b,a);
	if(a % b == 0) return b;
	else return gcd(b, a%b);
}

struct ModInt {
	ModInt() : mInt(-1) {};
	ModInt(ll n) : mInt(n) {};
	//~ModInt(){};
	ll mInt;
	static ll mMod;
	//todo
	ModInt& operator=(const ModInt& a) { mInt = a.mInt; return *this; }
	ModInt& operator+=(const ModInt& a) { mInt += a.mInt; mInt %= mMod; return *this; }
	ModInt& operator-=(const ModInt& a) { mInt -= a.mInt; if (mInt < 0) mInt += mMod; return *this; }
	ModInt& operator*=(const ModInt& a) { mInt *= a.mInt; mInt %= mMod; return *this; }
	ModInt operator+(const ModInt a) { return ModInt(mInt + a.mInt % mMod); }
	ModInt operator-(const ModInt a) { return ModInt(mInt - a.mInt < 0 ? mInt - a.mInt + mMod : mInt - a.mInt); }
	ModInt operator*(const ModInt a) { return ModInt(mInt * a.mInt % mMod); }
	ModInt& operator=(const ll& a) { mInt = a; return *this; }
	ModInt& operator+=(const ll& a) { mInt += a; mInt %= mMod; return *this; }
	ModInt& operator-=(const ll& a) { mInt -= a; if (mInt < 0) mInt += mMod; return *this; }
	ModInt& operator*=(const ll& a) { mInt *= a; mInt %= mMod; return *this; }
	ModInt operator+(const ll a) { return ModInt(mInt + a % mMod); }
	ModInt operator-(const ll a) { return ModInt(mInt - a < 0 ? mInt - a + mMod : mInt - a); }
	ModInt operator*(const ll a) { return ModInt(mInt * a % mMod); }
	ModInt operator++() { return ModInt(++mInt % mMod); }
	ModInt operator--() { return ModInt(++mInt % mMod); }
	bool operator==(ModInt a) { return mInt == a.mInt; }
	bool operator!=(ModInt a) { return mInt != a.mInt; }
	friend ostream& operator<<(ostream& os, const ModInt& n) {
		os << n.mInt;
		return os;
	}
	void setMod(ll mod) {
		ModInt::mMod = mod;
	}
};

ll ModInt::mMod = MOD;

ModInt pow(ModInt a, int b) {
	ModInt ret = 1;
	while (b > 0) {
		if (b & 1) ret *= a;
		a *= a;
		b >>= 1;
	}
	return ret;
}

ModInt inv(ModInt n) {
	return pow(n, n.mMod - 2);
}

ModInt fact(int n) {
	ModInt fac = 1;
	for (int i = n; i > 0; --i) fac *= i;
	return fac;
}

ModInt comb(ll n, ll r) {
	ModInt com = 1;
	if (n / 2 < r) return comb(n, n - r);
	else {
		for (int i = 0; i < r; ++i)
			com *= n - i;
		return com * inv(fact(r));
	}
}

void setComb(vector<ModInt>& combs, ll n, ll r) {
	//nCk = nCk-1 * n-k+1/k
	// combs[i] = nCi
	ModInt comb = 1;
	combs[0] = combs[n] = comb;
	if (n / 2 < r) setComb(combs, n, n - r);
	else {
		for (ll i = 1; i <= r; ++i) {
			comb *= inv(i) * (n - i + 1);
			combs[i] = combs[n - i] = comb;
		}
	}
}

void setComb(vector<ModInt>& combs, ll n) { setComb(combs, n, (n + 1) / 2); }

void setCombAll(vector<vector<ModInt>>& combs, ll n) {
	if (combs.size() < n - 1) cerr << "setCombAll : n" << n << " is small" << endl;
	rep(i, n + 1) rep(j, i + 1) {
		if (j == 0 || j == i) combs[i][j] = 1;
		else combs[i][j] = combs[i - 1][j - 1] + combs[i - 1][j];
	}
}


int main() {
	ll n; cin >> n ;

	vector<int> a(n); rep(i,n) cin >> a[i];

	vector<ll> sumodd(n + 1, 0), sumeven(n + 1,0);
	sumodd[0] = 0;
	rep(i, n) {
	  if (i % 2 == 0) { 
		  sumeven[i+1] = sumeven[i] + a[i];
			sumodd[i+1] = sumodd[i];
	  }
		else { 
		  sumodd[i + 1] = sumodd[i] + a[i];
			sumeven[i + 1] = sumeven[i];
		}
	}

	if (n % 2 == 0) {
		cout << max(sumodd[n], sumeven[n]) << endl;
		return 0;
	}
	else {
	  ll ans = 0;
		rep1(i, n) {
		  ll ans1 = 0, ans2 = 0;
			ans1 = sumodd[i-1]; ans2 = sumeven[i-1];

			if (i % 2 == 0) {
				if ((ll)i + 3 <= n) {
					if (i + 2 <= n)chmax(ans, ans1+sumodd[n]-sumodd[i+2]);
					if (i + 2 <= n)chmax(ans, ans2 + sumodd[n] - sumodd[i + 2]);
				}
			}
			else {
				if (i + 2 <= n)chmax(ans, ans1 + sumodd[n] - sumodd[i + 2]);
				if (i + 2 <= n)chmax(ans, ans2 + sumodd[n] - sumodd[i + 2]);
				if (i + 2 <= n)chmax(ans, ans1 + sumeven[n] - sumeven[i + 2]);
				if (i + 2 <= n)chmax(ans, ans2 + sumeven[n] - sumeven[i + 2]);
			}
		}

		cout << ans << endl;
	}
}

Compiler output:
main.c:1:10: fatal error: iostream: No such file or directory
    1 | #include <iostream>
      |          ^~~~~~~~~~
compilation terminated.