cf 498 B. Name That Tune

不会不会，，，%%http://hzwer.com/5813.html

 #include<cstdio>
 #include<iostream>
 #include<algorithm>
 #include<map>
 #include<cmath>
 #include<queue>
 #define N 1000005
 #define inf 1000000000
 #define LL long long
 using namespace std;
 int n,m;
 double ans,p1;
 double f[][];
 double p[],tmp[];
 int t[];
 int main()
 {
     scanf("%d%d",&n,&m);
     for (int i=; i<=n; i++)
     {
         int x,y; scanf("%d %d",&x,&t[i]);
         p[i]=(double)x/;
     }
     f[][]=; p[n+]=; t[n+]=;
     for (int i=; i<=n+; i++)
     {
         p1=pow(-p[i],t[i]);
         for (int j=; j<=m; j++) tmp[j]=f[i][j];
         for (int j=; j<=m; j++)
         {
             f[i][j+]+=f[i][j]*(-p[i]);
             f[i+][j+]+=f[i][j]*p[i];
             if (j+t[i]<=m)
             {
                 f[i][j+t[i]]-=tmp[j]*p1;
                 f[i+][j+t[i]]+=tmp[j]*p1;
             }
         }
     }
     for (int i=; i<=n+; i++) ans+=f[i][m]*(i-);
     printf("%.10lf",ans);
     return ;
 }