#include
using namespace std;
int main()
{
unsigned n, fib_i = 1, fib_i_plus1 = 1;
cin >> n;
if (n <= 2) cout << 1 << endl;
/* fib_i存储fib(i)的值，fib_i_plus1存储fib(i+1)的值。
* fib(i+1)=fib(i-1)+fib(i)，也就是上一次循环的fib_i_plus1+fib_i，
* fib(i)也就是上一次循环的fib_i */
for (unsigned i = 2; i != n; ++i) {
unsigned tmp = fib_i_plus1;
fib_i_plus1 += fib_i;
fib_i = tmp;
}
cout << fib_i_plus1 << endl;
return 0;
}
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