#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < n; ++i)
using ll = long long;
using namespace std;
const int INF = 1e9;

//https://dai1741.github.io/maximum-algo-2012/cpp/geometry/geometries2d.h

typedef double D;
typedef complex<D> P;
typedef pair<P, P> L;
typedef vector<P> VP;
const D EPS = 1e-9;
#define X real()
#define Y imag()
#define LE(n, m) ((n) < (m) + EPS)
#define GE(n, m) ((n) + EPS > (m))
#define EQ(n, m) (abs((n) - (m)) < EPS)

D dot(P a, P b) {
    return (conj(a) * b).X;
}
D cross(P a, P b) {
    return (conj(a) * b).Y;
}

int ccw(P a, P b, P c) {
    b -= a;
    c -= a;
    if (cross(b, c) > EPS)
        return +1;
    if (cross(b, c) < -EPS)
        return -1;
    if (dot(b, c) < -EPS)
        return +2;
    if (norm(b) < norm(c))
        return -2;
    return 0;
}

bool is_parallel(P a1, P a2, P b1, P b2) {
    P x = a2 - a1;
    P y = b2 - b1;
    return EQ(cross(x, y), 0.0);
}

bool is_orthogonal(P a1, P a2, P b1, P b2) {
    P x = a2 - a1;
    P y = b2 - b1;
    return EQ(dot(x, y), 0.0);
}

bool isecLP(P a1, P a2, P b) {
    return abs(ccw(a1, a2, b)) != 1;
}

bool isecLL(P a1, P a2, P b1, P b2) {
    return !isecLP(a2 - a1, b2 - b1, 0) || isecLP(a1, b1, b2);
}

bool isecLS(P a1, P a2, P b1, P b2) {
    return cross(a2 - a1, b1 - a1) * cross(a2 - a1, b2 - a1) < EPS;
}

bool isecSS(P a1, P a2, P b1, P b2) {
    return ccw(a1, a2, b1) * ccw(a1, a2, b2) <= 0 &&
           ccw(b1, b2, a1) * ccw(b1, b2, a2) <= 0;
}

bool isecSP(P a1, P a2, P b) {
    return !ccw(a1, a2, b);
}

P proj(P a1, P a2, P p) {
    return a1 + dot(a2 - a1, p - a1) / norm(a2 - a1) * (a2 - a1);
}

P reflection(P a1, P a2, P p) {
    return 2.0 * proj(a1, a2, p) - p;
}

D distLP(P a1, P a2, P p) {
    return abs(proj(a1, a2, p) - p);
}

D distLL(P a1, P a2, P b1, P b2) {
    return isecLL(a1, a2, b1, b2) ? 0 : distLP(a1, a2, b1);
}

D distLS(P a1, P a2, P b1, P b2) {
    return isecLS(a1, a2, b1, b2) ? 0 : min(distLP(a1, a2, b1), distLP(a1, a2, b2));
}

D distSP(P a1, P a2, P p) {
    P r = proj(a1, a2, p);
    if (isecSP(a1, a2, r))
        return abs(r - p);
    return min(abs(a1 - p), abs(a2 - p));
}

D distSS(P a1, P a2, P b1, P b2) {
    if (isecSS(a1, a2, b1, b2))
        return 0;
    return min(min(distSP(a1, a2, b1), distSP(a1, a2, b2)),
               min(distSP(b1, b2, a1), distSP(b1, b2, a2)));
}

P crosspointLL(P a1, P a2, P b1, P b2) {
    D d1 = cross(b2 - b1, b1 - a1);
    D d2 = cross(b2 - b1, a2 - a1);
    if (EQ(d1, 0) && EQ(d2, 0))
        return a1;
    if (EQ(d2, 0))
        throw "kouten ga nai";
    return a1 + d1 / d2 * (a2 - a1);
}

D distLC(P a1, P a2, P c, D r) {
    return max(distLP(a1, a2, c) - r, 0.0);
}

D distSC(P a1, P a2, P c, D r) {
    D dSqr1 = norm(c - a1), dSqr2 = norm(c - a2);
    if (dSqr1 < r * r ^ dSqr2 < r * r)
        return 0;
    if (dSqr1 < r * r & dSqr2 < r * r)
        return r - sqrt(max(dSqr1, dSqr2));
    return max(distSP(a1, a2, c) - r, 0.0);
}

VP crosspointLC(P a1, P a2, P c, D r) {
    VP ps;
    P ft = proj(a1, a2, c);
    if (!GE(r * r, norm(ft - c)))
        return ps;

    P dir = sqrt(max(r * r - norm(ft - c), 0.0)) / abs(a2 - a1) * (a2 - a1);
    ps.push_back(ft + dir);
    if (!EQ(r * r, norm(ft - c)))
        ps.push_back(ft - dir);
    return ps;
}

D distCC(P a, D ar, P b, D br) {
    D d = abs(a - b);
    return GE(d, abs(ar - br)) ? max(d - ar - br, 0.0) : abs(ar - br) - d;
}

VP crosspointCC(P a, D ar, P b, D br) {
    VP ps;
    P ab = b - a;
    D d = abs(ab);
    D crL = (norm(ab) + ar * ar - br * br) / (2 * d);
    if (EQ(d, 0) || ar < abs(crL))
        return ps;

    P abN = ab * P(0, sqrt(ar * ar - crL * crL) / d);
    P cp = a + crL / d * ab;
    ps.push_back(cp + abN);
    if (!EQ(norm(abN), 0))
        ps.push_back(cp - abN);
    return ps;
}

VP tangentPoints(P a, D ar, P p) {
    VP ps;
    D sin = ar / abs(p - a);
    if (!LE(sin, 1))
        return ps;
    D t = M_PI_2 - asin(min(sin, 1.0));
    ps.push_back(a + (p - a) * polar(sin, t));
    if (!EQ(sin, 1))
        ps.push_back(a + (p - a) * polar(sin, -t));
    return ps;
}

vector<L> tangentLines(P a, D ar, P b, D br) {
    vector<L> ls;
    D d = abs(b - a);
    rep(i, 2) {
        D sin = (ar - (1 - i * 2) * br) / d;
        if (!LE(sin * sin, 1))
            break;
        D cos = sqrt(max(1 - sin * sin, 0.0));
        rep(j, 2) {
            P n = (b - a) * P(sin, (1 - j * 2) * cos) / d;
            ls.push_back(L(a + ar * n, b + (1 - i * 2) * br * n));
            if (cos < EPS)
                break;
        }
    }
    return ls;
}

P circumcenter(P a, P b, P c) {
    a = (a - c) * 0.5;
    b = (b - c) * 0.5;
    return c + crosspointLL(a, a * P(1, 1), b, b * P(1, 1));
}

VP circlesPointsRadius(P a, P b, D r) {
    VP cs;
    P abH = (b - a) * 0.5;
    D d = abs(abH);
    if (d == 0 || d > r)
        return cs;
    D dN = sqrt(r * r - d * d);
    P n = abH * P(0, 1) * (dN / d);
    cs.push_back(a + abH + n);
    if (dN > 0)
        cs.push_back(a + abH - n);
    return cs;
}

VP circlesPointsTangent(P a, P b, P l1, P l2) {
    P n = (l2 - l1) * P(0, 1);
    P m = (b - a) * P(0, 0.5);
    D rC = dot((a + b) * 0.5 - l1, n);
    D qa = norm(n) * norm(m) - dot(n, m) * dot(n, m);
    D qb = -rC * dot(n, m);
    D qc = norm(n) * norm(m) - rC * rC;
    D qd = qb * qb - qa * qc;

    VP cs;
    if (qd < -EPS)
        return cs;
    if (EQ(qa, 0)) {
        if (!EQ(qb, 0))
            cs.push_back((a + b) * 0.5 - m * (qc / qb / 2));
        return cs;
    }
    D t = -qb / qa;
    cs.push_back((a + b) * 0.5 + m * (t + sqrt(max(qd, 0.0)) / qa));
    if (qd > EPS)
        cs.push_back((a + b) * 0.5 + m * (t - sqrt(max(qd, 0.0)) / qa));
    return cs;
}

P minEnclosingCircle(const VP &ps) {
    P c;
    double move = 0.5;
    rep(i, 39) {
        rep(t, 50) {
            D max = 0;
            int k = 0;
            rep(j, ps.size()) if (max < norm(ps[j] - c)) {
                max = norm(ps[j] - c);
                k = j;
            }
            c += (ps[k] - c) * move;
        }
        move /= 2;
    }
    return c;
}

namespace std {
bool operator<(const P a, const P b) {
    return a.X != b.X ? a.X < b.X : a.Y < b.Y;
}
} // namespace std

VP convexHull(VP ps) {
    int n = ps.size(), k = 0;
    sort(ps.begin(), ps.end());
    VP ch(2 * n);
    for (int i = 0; i < n; ch[k++] = ps[i++])
        while (k >= 2 && ccw(ch[k - 2], ch[k - 1], ps[i]) <= 0)
            --k;
    for (int i = n - 2, t = k + 1; i >= 0; ch[k++] = ps[i--])
        while (k >= t && ccw(ch[k - 2], ch[k - 1], ps[i]) <= 0)
            --k;
    ch.resize(k - 1);
    return ch;
}

bool isCcwConvex(const VP &ps) {
    int n = ps.size();
    rep(i, n) if (ccw(ps[i], ps[(i + 1) % n], ps[(i + 2) % n]) == -1) return false;
    return true;
}

int inConvex(P p, const VP &ps) {
    int n = ps.size();
    int dir = ccw(ps[0], ps[1], p);
    rep(i, n) {
        int ccwc = ccw(ps[i], ps[(i + 1) % n], p);
        if (!ccwc)
            return 2;
        if (ccwc != dir)
            return 0;
    }
    return 1;
}

int inCcwConvex(const VP &ps, P p) {
    int n = ps.size();
    P g = (ps[0] + ps[n / 3] + ps[n * 2 / 3]) / 3.0;
    if (g == p)
        return 1;
    P gp = p - g;

    int l = 0, r = n;
    while (l + 1 < r) {
        int mid = (l + r) / 2;
        P gl = ps[l] - g;
        P gm = ps[mid] - g;
        if (cross(gl, gm) > 0) {
            if (cross(gl, gp) >= 0 && cross(gm, gp) <= 0)
                r = mid;
            else
                l = mid;
        } else {
            if (cross(gl, gp) <= 0 && cross(gm, gp) >= 0)
                l = mid;
            else
                r = mid;
        }
    }
    r %= n;
    D cr = cross(ps[l] - p, ps[r] - p);
    return EQ(cr, 0) ? 2 : cr < 0 ? 0 : 1;
}

int inPolygon(const VP &ps, P p) {
    int n = ps.size();
    bool in = false;
    rep(i, n) {
        P a = ps[i] - p;
        P b = ps[(i + 1) % n] - p;
        if (EQ(cross(a, b), 0) && LE(dot(a, b), 0))
            return 2;
        if (a.Y > b.Y)
            swap(a, b);
        if ((a.Y * b.Y < 0 || (a.Y * b.Y < EPS && b.Y > EPS)) && LE(cross(a, b), 0))
            in = !in;
    }
    return in;
}

VP convexCut(const VP &ps, P a1, P a2) {
    int n = ps.size();
    VP ret;
    rep(i, n) {
        int ccwc = ccw(a1, a2, ps[i]);
        if (ccwc != -1)
            ret.push_back(ps[i]);
        int ccwn = ccw(a1, a2, ps[(i + 1) % n]);
        if (ccwc * ccwn == -1)
            ret.push_back(crosspointLL(a1, a2, ps[i], ps[(i + 1) % n]));
    }
    return ret;
}

pair<int, int> convexDiameter(const VP &ps) {
    int n = ps.size();
    int i = min_element(ps.begin(), ps.end()) - ps.begin();
    int j = max_element(ps.begin(), ps.end()) - ps.begin();
    int maxI, maxJ;
    D maxD = 0;
    rep(_, 2 * n) {
        if (maxD < norm(ps[i] - ps[j])) {
            maxD = norm(ps[i] - ps[j]);
            maxI = i;
            maxJ = j;
        }
        if (cross(ps[i] - ps[(i + 1) % n], ps[(j + 1) % n] - ps[j]) <= 0)
            j = (j + 1) % n;
        else
            i = (i + 1) % n;
    }
    return make_pair(maxI, maxJ);
}

D area(const VP &ps) {
    D a = 0;
    rep(i, ps.size()) a += cross(ps[i], ps[(i + 1) % ps.size()]);
    return a / 2;
}

P centroid(const VP &ps) {
    int n = ps.size();
    D aSum = 0;
    P c;
    rep(i, n) {
        D a = cross(ps[i], ps[(i + 1) % n]);
        aSum += a;
        c += (ps[i] + ps[(i + 1) % n]) * a;
    }
    return 1 / aSum / 3 * c;
}

VP voronoiCell(P p, const VP &ps, const VP &outer) {
    VP cl = outer;
    rep(i, ps.size()) {
        if (EQ(norm(ps[i] - p), 0))
            continue;
        P h = (p + ps[i]) * 0.5;
        cl = convexCut(cl, h, h + (ps[i] - h) * P(0, 1));
    }
    return cl;
}

struct Edge {
    int from, to;
    D cost;
    Edge(int from, int to, D cost) : from(from), to(to), cost(cost) {}
};
struct Graph {
    int n;
    vector<vector<Edge>> edges;
    Graph(int n) : n(n), edges(n) {}
    void addEdge(Edge e) {
        edges[e.from].push_back(e);
        edges[e.to].push_back(Edge(e.to, e.from, e.cost));
    }
};

Graph segmentArrangement(const vector<L> &segs, VP &ps) {
    int n = segs.size();
    rep(i, n) {
        ps.push_back(segs[i].first);
        ps.push_back(segs[i].second);
        rep(j, i) {
            if (isecSS(segs[i].first, segs[i].second, segs[j].first, segs[j].second))
                ps.push_back(crosspointLL(segs[i].first, segs[i].second, segs[j].first, segs[j].second));
        }
    }
    sort(ps.begin(), ps.end());
    ps.erase(unique(ps.begin(), ps.end()), ps.end());

    int m = ps.size();
    Graph gr(m);
    vector<pair<D, int>> list;
    rep(i, n) {
        list.clear();
        rep(j, m) {
            if (isecSP(segs[i].first, segs[i].second, ps[j]))
                list.push_back(make_pair(norm(segs[i].first - ps[j]), j));
        }
        sort(list.begin(), list.end());
        rep(j, list.size() - 1) {
            int a = list[j].second;
            int b = list[j + 1].second;
            gr.addEdge(Edge(a, b, abs(ps[a] - ps[b])));
        }
    }
    return gr;
}

Graph visibilityGraph(const VP &ps, const vector<VP> &objs) {
    int n = ps.size();
    Graph gr(n);
    rep(i, n) rep(j, i) {
        P a = ps[i], b = ps[j];
        if (!EQ(norm(a - b), 0))
            rep(k, objs.size()) {
                const VP &obj = objs[k];
                int inStA = inConvex(a, obj);
                int inStB = inConvex(b, obj);
                if ((inStA ^ inStB) % 2 || inStA * inStB != 1 && inConvex((a + b) * 0.5, obj) == 1)
                    goto skip;
                rep(l, obj.size()) {
                    P cur = obj[l];
                    P next = obj[(l + 1) % obj.size()];
                    if (isecSS(a, b, cur, next) && !isecSP(cur, next, a) && !isecSP(cur, next, b))
                        goto skip;
                }
            }
        gr.addEdge(Edge(i, j, abs(a - b)));
    skip : {}
    }
    return gr;
}
int main() {
    int N;
    cin >> N;
    VP p(N);
    rep(i, N) {
        int x, y;
        cin >> x >> y;
        p[i] = P(x, y);
    }

    printf("%.1f\n", area(p));

    return 0;
}


Measured memory_bytes: 3244