"use strict";
    
const main = arg => {
    arg = arg.trim().split("\n");
    const N = parseInt(arg[0]);
    const A = arg[1].split(" ").map(n=>parseInt(n));
    
    let answer = 0;
    
    for(let i=0; i<N; i++) {
        answer += 1 / A[i];
    }
    
    console.log(1 / answer);
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));