D.cpp

//Took help
#include<bits/stdc++.h>
#include<string>
using namespace std;
#define fastio ios_base::sync_with_stdio(false);cin.tie(NULL)

unsigned long long fac[100005];

/* Iterative Function to calculate (x^y)%p
in O(log y) */
unsigned long long power(unsigned long long x,
                                  unsigned long long y, unsigned long long p)
{
    unsigned long long res = 1; // Initialize result

    x = x % p; // Update x if it is more than or
    // equal to p

    while (y > 0)
    {

        // If y is odd, multiply x with result
        if (y & 1)
            res = (res * x) % p;

        // y must be even now
        y = y >> 1; // y = y/2
        x = (x * x) % p;
    }
    return res;
}

// Returns n^(-1) mod p
unsigned long long modInverse(unsigned long long n,
                                            unsigned long long p)
{
    return power(n, p - 2, p);
}

// Returns nCr % p using Fermat's little
// theorem.
unsigned long long nCrModPFermat(unsigned long long n,
                                 unsigned long long r, unsigned long long p)
{
    // If n<r, then nCr should return 0
    if (n < r)
        return 0;
    // Base case
    if (r == 0)
        return 1;

    // Fill factorial array so that we
    // can find all factorial of r, n
    // and n-r
    /*unsigned long long fac[n + 1];
    fac[0] = 1;
    for (int i = 1; i <= n; i++)
        fac[i] = (fac[i - 1] * i) % p;*/

    return (fac[n] * modInverse(fac[r], p) % p
            * modInverse(fac[n - r], p) % p)
           % p;
}
int main()
{
    fastio;
    unsigned long long n,k,sum=0,p=1000000007;
    cin>>n>>k;
    fac[0] = 1;
    for (int i = 1; i <= n; i++)
        fac[i] = (fac[i - 1] * i) % p;
    if(k>=n)
        cout<<power(2,n,p);
    else
    {
        for(int i=0;i<=k;i++)
            sum=(sum+nCrModPFermat(n,i,p))%p;

        cout<<sum%p<<"\n";
    }
    return 0;
}