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[5005][5005];
double p[5005],tmp[5005];
int t[5005];
int main()
{
    scanf("%d%d",&n,&m);
    for (int i=1; i<=n; i++) 
    {
        int x,y; scanf("%d %d",&x,&t[i]);
        p[i]=(double)x/100;
    }
    f[1][0]=1; p[n+1]=0; t[n+1]=5000;
    for (int i=1; i<=n+1; i++)
    {
        p1=pow(1-p[i],t[i]);
        for (int j=0; j<=m; j++) tmp[j]=f[i][j];
        for (int j=0; j<=m; j++)
        {
            f[i][j+1]+=f[i][j]*(1-p[i]);
            f[i+1][j+1]+=f[i][j]*p[i];
            if (j+t[i]<=m)
            {
                f[i][j+t[i]]-=tmp[j]*p1;
                f[i+1][j+t[i]]+=tmp[j]*p1;
            }
        }
    }
    for (int i=1; i<=n+1; i++) ans+=f[i][m]*(i-1);
    printf("%.10lf",ans);
    return 0;
}