import * as fs from 'node:fs';

const input = fs.readFileSync('/dev/stdin', 'utf8');
const lines = input.trim().split('\n');

const n = parseInt(lines[0]);
const a: number[] = [];
const b: number[] = [];

for (let i = 1; i <= n; i++) {
    const [aVal, bVal] = lines[i].split(' ').map(Number);
    a.push(aVal);
    b.push(bVal);
}

let result = -1;
const vec: number[] = [];

for (let i = 0; i < n; i++) {
    vec.push(b[i] - a[i]);
}

vec.sort((x, y) => x - y);

const first = vec[0];
const last = vec[vec.length - 1];

if (first === last && last > 0) {
    result = last;
}

console.log(result);