HUD 4576 Robot

#include <bits/stdc++.h>
using namespace std;
const int N = 1000001;
int l,r,n,m,w;
double dp[N][2];
double res;
int main(){
    ios::sync_with_stdio(false);
    while(cin>>n>>m>>l>>r){
        for (int i = 0; i < n; i++) {
            dp[i][0]=dp[i][1]=0;
        }
       //memset(dp,0,sizeof(dp)); 用这句TLE
        res=0;
        dp[0][0]=1;
        for (int i = 1; i <= m; i++) {
            cin>>w;
            for (int j = 0; j < n; j++) {
                dp[j][i%2]=(dp[(j+n-w)%n][!(i%2)]+dp[(j+w)%n][!(i%2)])/2;
            }
        }
        for (int i = l-1; i < r; i++) {
            res+=dp[i][m%2];
        }
        if (n==0&&m==0&&l==0&&r==0) continue;
        cout<<fixed<<setprecision(4)<<res<<endl;

    }
    return 0;
}