#include <bits/stdc++.h>
using namespace std;
template <class T>
inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
long long power(long long a, long long b) {
  long long res = 1;
  a = a % 1000000007;
  while (b > 0) {
    if (b & 1) {
      res = (res * a) % 1000000007;
      b--;
    }
    a = (a * a) % 1000000007;
    b >>= 1;
  }
  return res;
}
long long fermat_inv(long long y) { return power(y, 1000000007 - 2); }
long long gcd(long long a, long long b) { return (b == 0) ? a : gcd(b, a % b); }
long long cnt(long long n) {
  if (n <= 0) return 0;
  long long len = 0, tmp = n, ans = 0;
  while (n > 9) {
    n /= 10;
    len++;
  }
  len++;
  long long po = 1;
  if (n == 1) {
    for (long long p = 0; p < len - 1; p++) {
      ans += po;
      po *= 10;
    }
    ans += (tmp - po + 1);
  } else {
    for (long long p = 0; p < len; p++) {
      ans += po;
      po *= 10;
    }
  }
  return ans;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long tt, i, j, n, m, x, y, ans;
  tt = 1;
  while (tt--) {
    cin >> n;
    vector<long double> prob(n + 1);
    vector<vector<long double> > dp(n + 1, vector<long double>(n + 1, 0));
    for (long long p = 1; p < n + 1; p++) {
      cin >> x >> y;
      prob[p] = (cnt(y) - cnt(x - 1)) * 1.0 / (y - x + 1);
    }
    dp[0][0] = 1.0;
    for (long long p = 1; p < n + 1; p++) {
      dp[p][0] = (1 - prob[p]) * dp[p - 1][0];
      for (long long q = 1; q < p + 1; q++)
        dp[p][q] = prob[p] * dp[p - 1][q - 1] + (1 - prob[p]) * dp[p - 1][q];
    }
    long double res = 0.0;
    long double k;
    cin >> k;
    for (long long p = 0; p < n + 1; p++)
      if (100 * p >= n * k) res += (dp[n][p]);
    cout << fixed << setprecision(15) << res;
  }
  return 0;
}
