export {};

const square = (N: number, point: number[][]): string => {
    let temp = 0;
    for(let i=0; i<N; i++) {
        temp += point[i][0]*point[i+1][1] - point[i+1][0]*point[i][1];
    }
    return (0.5*Math.abs(temp)).toFixed(1);
}

const input = require("fs").readFileSync(0).toString().trim().split('\n');
const N = Number(input[0]);

let point: number[][] = [];
for(let i=1; i<=N; i++) point.push(input[i].split(" ").map(Number));
point.push(point[0]);

console.log(square(N, point));