#include<bits/stdc++.h>

#define LL long long

#define fi first

#define se second

#define mk make_pair

#define PLL pair<LL, LL>

#define PLI pair<LL, int>

#define PII pair<int, int>

#define SZ(x) ((int)x.size())

#define ull unsigned long long

using namespace std;

const int N =  + ;

const int inf = 0x3f3f3f3f;

const LL INF = 0x3f3f3f3f3f3f3f3f;

const int mod = 1e9 + ;

const double eps = 1e-;

const double PI = acos(-);

int n, T, t[N];

double dp[N][N], p[N], q[N], f[N];

int main() {

    scanf("%d%d", &n, &T);

    for(int i = ; i <= n; i++) {

        scanf("%lf%d", &p[i], &t[i]);

        p[i] /= ; q[i] =  - p[i];

        f[i] = pow(q[i], t[i]);

    }

    double ans = ;

    dp[][] = ;

    for(int i = ; i <= n; i++) {

        double gg = ;

        for(int j = ; j <= T; j++) {

            if(j < t[i]) gg = gg * q[i] + p[i] * dp[i - ][j - ];

            else if(j == t[i]) gg = gg * q[i] + p[i] * dp[i - ][j - ] + f[i] * dp[i - ][j - t[i]];

            else gg = gg * q[i] + p[i] * dp[i - ][j - ] - f[i] * dp[i - ][j - t[i] - ] + f[i] * dp[i - ][j - t[i]];

            dp[i][j] = gg;

            ans += dp[i][j];

        }

    }

    printf("%.12f\n", ans);

    return ;

}

/*

*/