35 lines
868 B
C++
35 lines
868 B
C++
#include<iostream>
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#include<cstdio>
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#include<cstring>
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using namespace std;
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const int N=220;
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int n,m,l,r;
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double dp[2][N];
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int main(){
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while(~scanf("%d%d%d%d",&n,&m,&l,&r)){
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if(n==0 && m==0 && l==0 && r==0)
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break;
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for(int i=0;i<=n;i++)
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dp[0][i]=0;
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dp[0][0]=1;
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int x,cur=0;
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while(m--){
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scanf("%d",&x);
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for(int i=0;i<n;i++)
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dp[cur^1][i]=0;
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for(int i=0;i<n;i++){
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if(dp[cur][i]==0)
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continue;
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dp[cur^1][((i-x)%n+n)%n]+=0.5*dp[cur][i];
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dp[cur^1][(i+x)%n]+=0.5*dp[cur][i];
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}
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cur^=1;
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}
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double ans=0;
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for(int i=l-1;i<r;i++)
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ans+=dp[cur][i];
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printf("%.4lf\n",ans);
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}
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return 0;
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}
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