n <- 100
k <- 2
# n×(n-1)×...×(n-k+1)の計算
a <- 1
for (i in (n - k + 1) : n) {
  a <- a * i
}
# k!の計算
b <- 1
for (i in 1 : k) {
  b <- b * i
}
# n!/(k!*(n-k)!)の計算
print(a / b)