#include <bits/stdc++.h>
#pragma comment(linker, "/STACK:268435456")
using namespace std;
template <typename T>
inline T abs(T a) {
  return ((a < 0) ? -a : a);
}
template <typename T>
inline T sqr(T a) {
  return a * a;
}
template <class T>
T gcd(T a, T b) {
  return a ? gcd(b % a, a) : b;
}
template <class T>
T lcm(T a, T b) {
  return a / gcd(a, b) * b;
}
template <class T>
T sign(T a) {
  return a > 0 ? 1 : (a < 0 ? -1 : 0);
}
const int dx[] = {-1, 0, 1, 0};
const int dy[] = {0, 1, 0, -1};
const int dxK[] = {-1, -1, 0, 1, 1, 1, 0, -1};
const int dyK[] = {0, 1, 1, 1, 0, -1, -1, -1};
const int dxKn[] = {-2, -1, 1, 2, 2, 1, -1, -2};
const int dyKn[] = {1, 2, 2, 1, -1, -2, -2, -1};
const int N = int(1e3) + 5;
const int M = int(3e2) + 9;
const int LOGN = 21;
const int SQN = 350;
const int MOD = int(1e9) + 7;
const int INF = int(1e9) + 100;
const long long INF64 = 2e18;
const double PI = double(3.1415926535897932384626433832795);
const double e = double(2.7182818284590452353602874713527);
const double EPS = 1e-9;
int n, k;
double p[N];
double dp[N][N];
bool u[N][N];
long long f(long long r) {
  int cnt = 0;
  long long res = 0;
  for (long long i = 1; i <= r; i *= 10, ++cnt) {
    if (cnt == 19) return res;
    long long L = i, R = i + i - 1;
    R = min(R, r);
    res += max(0LL, R - L + 1);
  }
  return res;
}
double f(long long l, long long r) {
  return double(f(r) - f(l - 1)) / (r - l + 1);
}
double calc(int pos, int cnt) {
  double &res = dp[pos][cnt];
  if (u[pos][cnt]) return res;
  u[pos][cnt] = true;
  res = 0;
  if (pos == n) return res = (cnt * 100 >= k * n);
  return res = (1 - p[pos]) * calc(pos + 1, cnt) +
               p[pos] * calc(pos + 1, cnt + 1);
}
void solve() {
  cin >> n;
  for (int i = 0; i < (int)(n); ++i) {
    long long l, r;
    cin >> l >> r;
    p[i] = f(l, r);
  }
  cin >> k;
  cout << calc(0, 0);
}
int main() {
  srand(time(NULL));
  cout << setprecision(25) << fixed;
  cerr << setprecision(25) << fixed;
  solve();
  return 0;
}
