#include<bits/stdc++.h>
using namespace std;
const int MAX=61;
int n,d;
long long dp[MAX];
int main()
{
    scanf("%d%d",&n,&d);
    long long l=(1ll<<(n-1))-1,r;
    dp[0]=0;
    for(int i=1;i<=n-d-1;++i)
    {
        if(i<=d+1) dp[i]=i;
        else dp[i]=dp[i-1]+dp[i-d-1]+1;
    }
    r=dp[max(0,n-d-1)];
    printf("%lld\n",l-r);
    return 0;
}