test

process.stdin.resume();
process.stdin.setEncoding('utf8');
// 自分の得意な言語で
// Let's チャレンジ！！

var chunks = ""
process.stdin.on('data', function (chunk) {
    chunks += chunk.toString();
});
process.stdin.on('end', function () {
    var line = chunks.split(/\r?\n/g);
    var all = 0, temp, i, max = 0;
    temp = line[0].split(/\s/);
    line = line.slice(1, 1 + temp[1]);
    for (i = 0; i < line.length; i++) {
        all = all + (+line[i]) - ((+line[i - temp[0]]) || 0)
        max = max > all ? max : all;
    }
    console.log(max);
});