function Main(input) {
  const data = input.split("\n");
  const d = Number(data[0]);
  const c = data[1] + data[2];
  let max = 0;
  for(let i=0; i<c.length-d; i++) {
    if(haveLength(c, i, d)) {
      const len = countBefor(c, i-1) + d + countAfter(c, i+d);
      max = Math.max(max, len);
    }
  }
  console.log(max);
}

function haveLength(list, start, length) {
  for(let i=start; i<start+length; i++) {
    if(list[i] == 'o') return false;
  }
  return true;
}

function countAfter(list, start) {
  let count = 0;
  for(let i=start; i<list.length; i++) {
    if(list[i] == 'x') return count;
    count++;
  }
  return count;
}

function countBefor(list, start) {
  let count = 0;
  for(let i=start; i>=0; i--) {
    if(list[i] == 'x') return count;
    count++;
  }
  return count;
}

Main(require("fs").readFileSync("/dev/stdin", "utf8"));