#include <bits/stdc++.h>
using namespace std;
const double pi = acos(-1.0);
const double eps = 1e-9;
const int inf = ~(1 << 31);
const int N = 1000000;
int luck[N], top = 1;
char s[16];
void gen(int i) {
  s[i] = '4';
  if (i == 0) {
    stringstream ss;
    ss << s;
    int k;
    ss >> k;
    luck[top++] = k;
  } else
    gen(i - 1);
  s[i] = '7';
  if (i == 0) {
    stringstream ss;
    ss << s;
    int k;
    ss >> k;
    luck[top++] = k;
  } else
    gen(i - 1);
  s[i] = 0;
}
double foo(double a, double b, double c, double d) {
  if (c <= a && d >= b) return b - a + 1;
  if (c >= a && d <= b) return d - c + 1;
  if (d >= a && c <= a) return d - a + 1;
  if (d >= b && c <= b) return b - c + 1;
  return 0;
}
int main() {
  int i, n, t;
  luck[0] = 0;
  for (i = 0; i < 9; i++) gen(i);
  luck[top] = inf;
  sort(luck, luck + top);
  double ul, ur, vl, vr;
  int k;
  cin >> ul >> ur >> vl >> vr >> k;
  double r = 0;
  for (i = 1; i + k - 1 < top; i++) {
    int j = i + k - 1;
    r += foo(luck[i - 1] + 1, luck[i], ul, ur) *
         foo(luck[j], luck[j + 1] - 1, vl, vr);
    r += foo(luck[i - 1] + 1, luck[i], vl, vr) *
         foo(luck[j], luck[j + 1] - 1, ul, ur);
  }
  if (k == 1) {
    for (i = 1; i < top; i++) {
      k = luck[i];
      if (k >= vl && k <= vr && k >= ul && k <= ur) r--;
    }
  }
  r /= double(vr - vl + 1);
  r /= double(ur - ul + 1);
  if (r > 1 + eps) r = 1.0;
  printf("%.9lf", r);
  return 0;
}
