#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 ;

 }