paiza.js

const stream = require('fs').readFileSync('/dev/stdin', 'utf8').trim()
const cols = stream.split(' ')
const lines = stream.split('\n')
const matrix = lines.map(line => line.split(' '))

const fn = (N, C, matrix) => {
  const f = (acc, [n, c]) => [
    ...acc.slice(0, n - 1), c, ...acc.slice(n)
  ]
  const pairs = matrix.map(([n, c]) => [Number(n), c])
  return pairs.reduce(f, Array(Number(N)).fill(C)).join('')
}

const [[N], , ...rest] = matrix
const [ns, [C]] = [rest.slice(0, -1), rest.at(-1)]

const result = fn(N, C, ns)

console.log(result)