function fibonacci(n) {
    if(n === 1 || n === 2) {
        return 1
    } else {
        let pre2 = 1
        let pre1 = 1
        let summary = 0
        for(let i = 3; i <= n; i++) {
            summary = pre2 + pre1
            pre2 = pre1
            pre1 = summary
        }
        return summary
    }
}