一本通1652：牡牛和牝牛

#include <cstdio>
#define LL long long

const int mod = 5000011;

LL qmi(LL a, int b) {
    LL res = 1;

    while (b) {
        if (b & 1)
            res = res * a % mod;

        a = a * a % mod;
        b >>= 1;
    }

    return res;
}

LL C(LL a, LL b) {
    if (a < b)
        return 0;

    LL x = 1, y = 1;

    for (int i = a, j = 1; j <= b; j ++, i --)
        x = x * i % mod, y = y * j % mod;

    return x * qmi(y, mod - 2) % mod;
}

int main() {
    int n, k;
    LL ans = 1;
    scanf("%d %d", &n, &k);

    for (int i = 1; i + (i - 1) * k <= n; i ++)
        ans += C(n - (i - 1) * k, i), ans %= mod;

    printf("%lld", ans);
    return 0;
}