export {};
const input = require("fs").readFileSync(0).toString().trim().split('\n');
const [n, m]: number[] = input[0].split(" ").map(Number);

let parent: number[] = Array.from({length: n}, (_, i) => i);
let rank: number[] = new Array(n).fill(0);

const find = (x: number): number => {
    if(x !== parent[x]) return parent[x] = find(parent[x]);
    return parent[x];
}

const union = (x: number, y: number): boolean => {
    x = find(x);
    y = find(y);

    if(x===y) return false;

    if(rank[x] < rank[y]) [x, y] = [y, x];

    parent[y] = x;
    if(rank[x] === rank[y]) rank[x] += 1;

    return true;
}

let ans: number = 0;
for(let i=1; i<=m; i++) {
    let [u, v]: number[] = input[i].split(" ").map(Number);
    if(!union(u, v)) {
        ans = i;
        break;
    }
}

console.log(ans);