Problem:
Score : 800 points

Problem Statement

Snuke has a string x of length N.
Initially, every character in x is 0.

Snuke can do the following two operations any number of times in any order:

Choose A consecutive characters in x and replace each of them with 0.

Choose B consecutive characters in x and replace each of them with 1.

Find the number of different strings that x can be after Snuke finishes doing operations.
This count can be enormous, so compute it modulo (10^9+7).

Constraints

1 \leq N \leq 5000

1 \leq A,B \leq N

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N A B

Output

Print the number of different strings that x can be after Snuke finishes doing operations, modulo (10^9+7).

Sample Input 1

4 2 3

Sample Output 1

11

For example, x can be 0011 or 1111 in the end, but cannot be 0110.

Sample Input 2

10 7 2

Sample Output 2

533

Sample Input 3

1000 100 10

Sample Output 3

828178524

Solution in C++ (GCC 9.2.1) (judge verdict: Accepted)

#define ENABLE_DEBUG 0
// Kana's kitchen {{{
#include<bits/stdc++.h>
#define ALL(v) std::begin(v),std::end(v)
#define LOOP(k) for(i64 ngtkana_is_a_genius=0; ngtkana_is_a_genius<(i64)k; ngtkana_is_a_genius++)

using i32 = std::int_least32_t;
using i64 = std::int_least64_t;
using u32 = std::uint_least32_t;
using u64 = std::uint_least64_t;
using usize = std::size_t;

template <class T, class U> using pair = std::pair<U, T>;
template <class T> using diag_pair = std::pair<T, T>;
template <class... Args> using tuple = std::tuple<Args...>;
template <class T> using vec = std::vector<T>;
template <class T> using numr = std::numeric_limits<T>;

#ifdef NGTKANA
#include<debug.hpp>
#else
#define DEBUG(...)(void)0
#endif
/*}}}*/
// mint{{{
template <class ModType> struct modint {
    using value_type = typename ModType::value_type;
    using mint = modint<ModType>;
    using mod_type = ModType;

    static value_type mod() { return ModType::value; }

    private:
    static value_type inverse(value_type x) {
        value_type y=1,u=mod(),v=0;
        while(x){
            value_type q=u/x;
            u-=q*x; std::swap(x,u);
            v-=q*y; std::swap(y,v);
        }
        assert(x==0 && std::abs(y)==mod() && std::abs(u)==1 && std::abs(v)<mod());
        return v<0?v+mod():v;
    }

    public:
    // the member variable
    value_type value;

    // constructors
    modint()=default;
    modint(modint const&)=default;
    modint(modint&&)=default;
    modint& operator=(modint const&)=default;
    modint& operator=(modint&&)=default;
    ~modint()=default;

    template <class T> modint(T t) : value([t] () mutable {
            if ( t <= -static_cast<T>(mod()) || static_cast<T>(mod()) <= t ) t %= mod();
            return t < 0 ? t + mod() : t;
            }()) {}

    // operators
    mint& operator+= (mint y) {
        value += y.value;
        if (mod() <= value) value -= mod();
        return *this;
    }

    mint& operator-= (mint y) {
        value -= y.value;
        if ( value < 0 ) value += mod();
        return *this;
    }

    mint& operator*= (mint y) {
        value = (long long)value * y.value % mod();
        return *this;
    }

    mint& operator/= (mint y) {
        value = (long long)value * inverse(y.value) % mod();
        return *this;
    }

    mint& operator++() { return *this+=1; }
    mint& operator--() { return *this-=1; }
    mint operator++(int) { mint this_=*this; ++*this; return this_; }
    mint operator--(int) { mint this_=*this; --*this; return this_; }
    mint operator-() const { return 0 - *this; }

    // static member functions
    static mint inv(mint x) { return inverse(x.value); }

    static mint m1pow(long long y) { return y%2?-1:1; }

    static mint pow(mint x, unsigned long long y) {
        mint ans=1;
        for(;y;y>>=1){
            if(y&1ull) ans*=x;
            x*=x;
        }
        return ans;
    }

    // non-member functions
    mint& add_assign(mint y) { return *this+=y; }
    mint& sub_assign(mint y) { return *this-=y; }
    mint& mul_assign(mint y) { return *this*=y; }
    mint& div_assign(mint y) { return *this/=y; }
    mint& inv_assign()       { return *this = inv(*this); }
    mint& pow_assign(unsigned long long y){ return *this = pow(*this, y); }

    mint add(mint y) const { mint ans=*this; return ans.add_assign(y); }
    mint sub(mint y) const { mint ans=*this; return ans.sub_assign(y); }
    mint mul(mint y) const { mint ans=*this; return ans.mul_assign(y); }
    mint div(mint y) const { mint ans=*this; return ans.div_assign(y); }
    mint inv()       const { mint ans=*this; return ans.inv_assign(); }
    mint pow(unsigned long long y) const { return pow(*this, y); }
    mint square(mint x) const { return *this * *this; }
    mint cube(mint x) const { return *this * *this * *this; }

    template <class F> mint map(F const& f){
        value=f(value);
        return *this;
    }
};

template <class T> std::istream&
operator>>(std::istream& is, modint<T>& x) {
    typename modint<T>::value_type y;
    is >> y;
    x = modint<T>{ y };
    return is;
}
template <class T> std::ostream&
operator<<(std::ostream& os, modint<T> x) {
    return os << x.value;
}

template <class T> modint<T> operator+(modint<T> x, modint<T> y) { return x+=y; }
template <class T> modint<T> operator-(modint<T> x, modint<T> y) { return x-=y; }
template <class T> modint<T> operator*(modint<T> x, modint<T> y) { return x*=y; }
template <class T> modint<T> operator/(modint<T> x, modint<T> y) { return x/=y; }
template <class T> bool operator==(modint<T> x, modint<T> y) { return x.value==y.value; }
template <class T> bool operator!=(modint<T> x, modint<T> y) { return x.value!=y.value; }

template <class T, class U> modint<T> operator+(modint<T> x, U y) { return x+modint<T>(y); }
template <class T, class U> modint<T> operator-(modint<T> x, U y) { return x-modint<T>(y); }
template <class T, class U> modint<T> operator*(modint<T> x, U y) { return x*modint<T>(y); }
template <class T, class U> modint<T> operator/(modint<T> x, U y) { return x/modint<T>(y); }
template <class T, class U> bool operator==(modint<T> x, U y) { return x==modint<T>(y); }
template <class T, class U> bool operator!=(modint<T> x, U y) { return x!=modint<T>(y); }

template <class T, class U> modint<T> operator+(U x, modint<T> y) { return modint<T>(x)+y; }
template <class T, class U> modint<T> operator-(U x, modint<T> y) { return modint<T>(x)-y; }
template <class T, class U> modint<T> operator*(U x, modint<T> y) { return modint<T>(x)*y; }
template <class T, class U> modint<T> operator/(U x, modint<T> y) { return modint<T>(x)/y; }
template <class T, class U> bool operator==(U x, modint<T> y) { return modint<T>(x)==y; }
template <class T, class U> bool operator!=(U x, modint<T> y) { return modint<T>(x)!=y; }
/*}}}*/
using mint = modint<std::integral_constant<i64, 1'000'000'007>>;
template <usize N, class T, class... Args, std::enable_if_t<N==0, int> = 0>
auto mkvec(Args... args) {
    return T(args...);
}
template <usize N, class T, class... Args, std::enable_if_t<N!=0, int> = 0>
auto mkvec(usize sz, Args... args) {
    using value_type = std::decay_t<decltype(mkvec<N-1, T>(args...))>;
    return vec<value_type>(sz, mkvec<N-1, T>(args...));
}
int main() {
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);
    std::cout << std::setprecision(15) << std::fixed;

    usize n, a, b;
    std::cin >> n >> a >> b;
    if (a>b) std::swap(a, b);

    auto fp = mkvec<2, mint>(2, b);
    fp.at(0).at(0) = 1;
    fp.at(1).at(0) = 1;
    for (usize i=0; i<b; i++) {
        for (usize j=a; i+j<b; j++) fp.at(0).at(i+j) += fp.at(1).at(i);
        for (usize j=1; i+j<b; j++) fp.at(1).at(i+j) += fp.at(0).at(i);
    }

    vec<mint> ep(b);
    ep.at(0) = 1;
    for (usize i=1; i<b; i++) {
        ep.at(i) = fp.at(0).at(i) + fp.at(1).at(i);
    }

    auto dp = mkvec<2, mint>(2, n);
    for (usize j=1; j<a; j++) dp.at(0).at(j) += 1;
    for (usize j=1; j<b; j++) dp.at(1).at(j) += ep.at(j-1);

    for (usize i=1; i<n; i++) {
        for (usize j=1; j<a && i+j<n; j++) dp.at(0).at(i+j) += dp.at(1).at(i);
        for (usize j=1; j<b && i+j<n; j++) dp.at(1).at(i+j) += dp.at(0).at(i) * ep.at(j==1?1:j-2);
    }

    mint ans = 0;
    for (usize j=1; j<b; j++) ans += dp.at(0).at(n-j) * ep.at(j-1);
    for (usize j=1; j<a; j++) ans += dp.at(1).at(n-j);
    ans = mint::pow(2, n) - ans;
    std::cout << ans << '\n';
}