Go: package main

import "fmt"

func main() {

	for true {
		var n, k int
		fmt.Scan(&n, &k)

		if n == 0 && k == 0 {
			return
		}
		
		a := make([]int, n)
		ans := make([]int, n)
		for i := 0; i < n; i++ {
			fmt.Scan(&a[i])
			if i == 0 {
				ans[i] = a[i]
			} else {
				ans[i] = ans[i-1] + a[i]
			}
		}
		max := 0
		for i := k; i < n; i++ {
			t := ans[i] - ans[i-k]
			if max < t {
				max = t
			}
		}
		fmt.Println(max)
	}
}

C++: #include <iostream>
#include <array>
#include <vector>
#include <map>
#include <unordered_map>
#include <set>
#include <unordered_set>
#include <algorithm>
#include <math.h>
#include <string>
#include <climits>
#include <cassert>
#include <iomanip>
#include <bitset>
#include <queue>
#include <deque>
#include <stack>
#include <functional>
#include <fstream>
#include <numeric>

#define FOR(i, a, n) for(int i=(a);i<(n); ++i)
#define FOE(i, a) for(auto i : a)
#define ALL(c) (c).begin(), (c).end()
#define RALL(c) (c).rbegin(), (c).rend()
#define SUM(x) std::accumulate(ALL(x), 0LL)
#define MIN(v) *std::min_element(v.begin(), v.end())
#define MAX(v) *std::max_element(v.begin(), v.end())
#define EXIST(v, x) (std::find(v.begin(), v.end(), x) != v.end())
#define IS_BIT_ON(bit, i) (bit & (1 << i))
#define BIT_ON(bit, i) (bit |= (1 << i))
#define BIT_OFF(bit, i) (bit &= ~(1 << i))
#define BIT_COUNT(bit) (__builtin_popcount(bit))

typedef long long LL;

template<typename T>
std::vector<T> make_v(size_t a) { return std::vector<T>(a); }

template<typename T, typename... Ts>
auto make_v(size_t a, Ts... ts) { return std::vector<decltype(make_v<T>(ts...))>(a, make_v<T>(ts...)); }    // C++14
template<typename T, typename V>
typename std::enable_if<std::is_class<T>::value == 0>::type fill_v(T &t, const V &v) { t = v; }

template<typename T, typename V>
typename std::enable_if<std::is_class<T>::value != 0>::type fill_v(T &t, const V &v) { for (auto &e:t) fill_v(e, v); }

template<class T>
inline T ceil(T a, T b) { return (a + b - 1) / b; }

void print() { std::cout << std::endl; }

template<class Head, class... Tail>
void print(Head &&head, Tail &&... tail) {
    std::cout << head;
    if (sizeof...(tail) != 0) { std::cout << " "; }
    print(std::forward<Tail>(tail)...);
}

template<class T>
void print(std::vector<T> &v) {
    for (auto &a : v) {
        std::cout << a;
        if (&a != &v.back()) { std::cout << " "; }
    }
    std::cout << std::endl;
}

template<class T>
void print(std::vector<std::vector<T>> &vv) { for (auto &v : vv) { print(v); }}

void debug() { std::cerr << std::endl; }

template<class Head, class... Tail>
void debug(Head &&head, Tail &&... tail) {
    std::cerr << head;
    if (sizeof...(tail) != 0) { std::cerr << " "; }
    print(std::forward<Tail>(tail)...);
}

template<class T>
void debug(std::vector<T> &v) {
    for (auto &a : v) {
        std::cerr << a;
        if (&a != &v.back()) { std::cerr << " "; }
    }
    std::cerr << std::endl;
}

template<class T>
void debug(std::vector<std::vector<T>> &vv) { for (auto &v : vv) { print(v); }}

inline bool inside(long long y, long long x, long long H, long long W) { return 0 <= y and y < H and 0 <= x and x < W; }

template<class T>
inline double euclidean_distance(T y1, T x1, T y2, T x2) { return sqrt((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2)); }

template<class T>
inline double manhattan_distance(T y1, T x1, T y2, T x2) { return abs(x1 - x2) + abs(y1 - y2); }

template<typename T>
T &chmin(T &a, const T &b) { return a = std::min(a, b); }

template<typename T>
T &chmax(T &a, const T &b) { return a = std::max(a, b); }

template<class T>
inline std::vector<T> unique(std::vector<T> v) {
    sort(v.begin(), v.end());
    v.erase(unique(v.begin(), v.end()), v.end());
    return v;
}

long long sum_of_arithmetic_progression(long long s, long long d, long long n) {
    return n * (2 * s + (n - 1) * d) / 2;
}

bool is_power_of_two(long long x) {
    return !(x & (x - 1));
}

long long gcd(long long a, long long b) {
    if (b == 0) { return a; }
    return gcd(b, a % b);
}

long long gcd(std::vector<long long> &v) {
    long long ans = v[0];
    for (int i = 1; i < (int) v.size(); ++i) {
        ans = gcd(ans, v[i]);
    }
    return ans;
}

long long lcm(long long a, long long b) {
    long long g = gcd(a, b);
    return a / g * b;
}

const int INF = 1u << 30u;
const long long LINF = 1ull << 58u;
const double EPS = 1e-9;
const long double PI = acos(-1.0);
const std::vector<int> dy2 = {0, 1}, dx2 = {1, 0};
const std::vector<int> dy4 = {0, 1, 0, -1}, dx4 = {1, 0, -1, 0};
const std::vector<int> dy8 = {0, -1, 0, 1, 1, -1, -1, 1}, dx8 = {1, 0, -1, 0, 1, 1, -1, -1};

template<typename T> class CumulativeSum {
public:
    std::vector<T> memo1;               // 1次元用の累積和表
    std::vector<std::vector<T>> memo2;  // 2次元用の累積和表

    CumulativeSum() {}

    CumulativeSum(const std::vector<T> &line) {
        this->build(line);
    }

    CumulativeSum(const std::vector<std::vector<T>> &matrix) {
        this->build(matrix);
    }

    void build(const std::vector<T> &line) {
        this->memo1 = std::vector<T>(line.size() + 1, 0);
        for (int i = 0; i < line.size(); ++i) {
            this->memo1[i + 1] = this->memo1[i] + line[i];
        }
    }

    // 2次元の累積和表を作成
    void build(const std::vector<std::vector<T>> &board) {
        const int height = board.size();
        const int width = board[0].size();

        this->memo2 = std::vector<std::vector<T>>(height + 1, std::vector<T>(width + 1));

        for (int y = 0; y < height; ++y) {
            for (int x = 0; x < width; ++x) {
                this->memo2[y + 1][x + 1] = board[y][x] + this->memo2[y + 1][x];
            }
            for (int x = 0; x < width; ++x) {
                this->memo2[y + 1][x + 1] += this->memo2[y][x + 1];
            }
        }
    }

    // sum[left, right)
    T sum(int left, int right) {
        assert(right >= left);
        return this->memo1[right] - this->memo1[left];
    }

    /*
    左上(y1, x1)から右下(y2, x2)の合計を返す．(y2, x2)の行と列は含まない
    座標はmemoを作成したboard準拠
    (ex, sum(0, 0, 2, 2)なら(0, 0), (0, 1), (1, 0), (1, 1)の合計を返す)
    */
    T sum(int y1, int x1, int y2, int x2) {
        assert(y2 >= y1);
        assert(x2 >= x1);
        return this->memo2[y2][x2] - this->memo2[y2][x1] - this->memo2[y1][x2] + this->memo2[y1][x1];
    }
};

using namespace std;


int main(int argc, char *argv[]) {
    cin.tie(0);
    ios::sync_with_stdio(false);

    while (true) {
        LL N, K;
        cin >> N >> K;
        if (N == 0 and K == 0) {
            break;
        }

        vector<long long> v(N);
        FOR(i, 0, N) {
            cin >> v[i];
        }

        CumulativeSum<long long> cs(v);
        long long ans = LONG_LONG_MIN;
        for (int i = 0; i < N - K + 1; ++i) {
            ans = max(ans, cs.sum(i, i + K));
        }

        cout << ans << endl;
    }

    return 0;
}

C++: #include <iostream>
#include <algorithm>
#include <vector>
#include <cstdio>
#include <string>
#include <cmath>
#include <cfloat>
#include <map>
using namespace std;
int main(){

	int n,k;
	while(cin>>n>>k,n!=0||k!=0){
	int s=-10001;
	vector<int> a(n);
	
	for(int i=0;i<n;i++)
		cin>>a[i];
		
		for(int i=0;i+k<n;i++){
			int y=0;
			for(int j=i;j<i+k;j++){
				y+=a[j];
			if(s<y)
				s=y;
			}
		}
		cout<<s<<endl;
	}
	
}

C++: #include <algorithm>
#include <cstdint>
#include <iostream>
#include <vector>

using namespace std;

int main() {
  int n, k;
  cin >> n >> k;
  vector<int> a(n);
  vector<int64_t> s(n + 1);
  for (int i = 0; i < n; i++) {
    cin >> a[i];
    s[i + 1] = s[i] + a[i];
  }
  int64_t ans = 0;
  for (int i = 0; i < n - k + 1; i++) {
    ans = max(ans, s[i + k] - s[i]);
  }
  cout << ans << endl;
  return 0;
}