process.stdin.resume();
process.stdin.setEncoding('utf8');
// 自分の得意な言語で
// Let's チャレンジ！！
process.stdin.on('data', function (chunk) {
    var lines = chunk.toString().split('\n');
    var param = lines.shift().split(' ').map(Number);
    var n = param[0];
    var m = param[1];
    var data = lines.slice(0,m).join(' ').split(' ').map(Number);
    var seats = [];

    for (var i =0;i<n;i++) {
        seats.push(0);
    }

    function isVachant(begin,end,array) {
        var checkArray = [];
        for (var i= begin-1; i < end-1 ;i++ ) {
            checkArray.push(array[i % array.length]);
        }

        return checkArray.every(function (v) {
            return v === 0;
        });
    }

    for (var ix =0;ix<m*2;ix+=2){
        var ary1 = data[ix+1];
        var ary2 = data[ix];
        if (isVachant(ary1,ary1+ary2,seats) === true) {
            addPerson(ary1,ary1+ary2,seats);
        }
    }

    function addPerson(b,e,array){
        for (var i=b-1 ; i < e -1 ;i++) {
            array[i % array.length] = 1;
        }
        return array;
    }

    var count=0;
    for (var index=0;index<seats.length;index++) {
        if (seats[index] == 1) {
            count++;
        }
    }
    console.log(count);
});