#include <iostream>
#include <vector>
#include <set>
#include <algorithm>
using namespace std;

void generatePermutations(string &s, int index, set<string> &result) {
    if (index == s.size()) {
        result.insert(s);  
        return;
    }
    
    for (int i = index; i < s.size(); ++i) {
        swap(s[index], s[i]);
        generatePermutations(s, index + 1, result);  
        swap(s[index], s[i]);  
    }
}

vector<string> solve(string s) {
    if (s.length() < 0 || s.length() > 5 || !all_of(s.begin(), s.end(), ::islower)) {
        return {};
    }

    set<string> result; 
    sort(s.begin(), s.end()); 
    generatePermutations(s, 0, result); 
    
    vector<string> resultVector(result.begin(), result.end());  
    return resultVector;
}

def solve(rectangles):
    covered = set()
    for width, height in rectangles:
        for x in range(width):
            for y in range(height):
                covered.add((x, y)) 
    return len(covered)

def main():
    n = int(input())
    rectangles = []
    for _ in range(n):
        width, height = map(int, input().split())
        rectangles.append((width, height))
    result = solve(rectangles)
    print(result)
if __name__ == "__main__":
    main()

int fun(int node, int parent, vector<vector<int> >&adj, vector<int>&weight, int &maxi)
{
    int sum1 = 0;
    for(auto it:adj[node])
    {
        if(it != parent)
        {
            sum1 += fun(it,node,adj,weight,maxi);
        }
    }

    sum1 = max(sum1,weight[node-1]);
    maxi = max(maxi,sum1);
    return sum1;
}

int findMaximumSum(int tree_nodes, vector<int> tree_from, vector<int> tree_to, vector<int> weight)
{
    int n = tree_nodes;

    vector<vector<int> > adj(n+1);
    for(int i = 0; i<tree_from.size(); i++)
    {
        adj[tree_from[i]].push_back(tree_to[i]);
        adj[tree_to[i]].push_back(tree_from[i]);
    }

    int maxi = -1e9;
    fun(1,-1,adj,weight,maxi);
    return maxi;

}