#include <stdio.h>
#include <math.h>

int main() {
  int i = 0;
  double lg100f = 0.0;
  for(i = 2; i <= 100; ++i) {
    lg100f += log10(i);
  }
  printf("100! has %g digits.\n", ceil(lg100f));
  return 0;
}

100! has 158 digits.