#include <bits/stdc++.h>

#ifndef LOCAL_
#define fprintf if( false ) fprintf
#endif // LOCAL_
#define dumpi(x1) fprintf(stderr, "#%s.%d (%s) = (%d)\n", __func__, __LINE__, #x1, x1);
#define dumpii(x1, x2) fprintf(stderr, "#%s.%d (%s, %s) = (%d, %d)\n", __func__, __LINE__, #x1, #x2, x1, x2);
#define dumpiii(x1, x2, x3) fprintf(stderr, "#%s.%d (%s, %s, %s) = (%d, %d, %d)\n", __func__, __LINE__, #x1, #x2, #x3, x1, x2, x3);
#define dumpl(x1) fprintf(stderr, "#%s.%d (%s) = (%ld)\n", __func__, __LINE__, #x1, x1);
#define dumpd(x1) fprintf(stderr, "#%s.%d (%s) = (%lf)\n", __func__, __LINE__, #x1, x1);

int h, a, d;
double es[11234];

double e(int x) {
  if( x <= 0 ) return 0.0;
  return es[x];
}

int main() {
  scanf("%d %d %d", &h, &a, &d);
  for(int i = 1; i <= h; ++i) {
    double base = 0.0;
    double min = 10000000;
    for(int j = 0; j < 100; ++j) {
      if( j > 0 ) base += pow(1.0/3.0, j) * (j + e(i-d));
      double t = 2.0 * base + pow(1.0/3.0, j) * ((j + 1) + e(i-a));
      min = std::min(min, t);
    }
    dumpd(min);
    es[i] = min;
  }
  printf("%.20lf\n", es[h]);

  return 0;
}