function plus(a, b) {
  minus(a, minus(0, b))
}

function times(a, b) {
  if (eq(b, 0)) {
    0
  } else {
    plus(a, times(a, minus(b, 1)))
  }
}

function eq(a, b) {
  if (minus(a, b)) {
    0
  } else {
    1
  }
}

function factorial(n) {
  if (eq(n, 1)) {
    1
  } else {
    times(n, factorial(minus(n, 1)))
  }
}

print(factorial(4))