C :

#include <stdio.h>
int n, a, b, i;
double t;
int main(void) {
    scanf("%d", &n);
    for (i = 1; i <= n; i++) {
        scanf("%d %d", &a, &b);
        t += a * 0.1 * b;
    }
    printf("%.2f\n", t);
    return 0;
}

C++ :

#include<iostream>
#include<cstdio>
using namespace std;

int main()
{
	double n,a,b,s;
	cin>>n;
	s=0;
	for(int i=1;i<=n;i++)
	{
		cin>>a>>b;
		s=s+b/10*a;
	}
	printf("%.2lf",s);
	return 0;
}

Python :

# coding=utf-8
n = int(input())
t = 0
for i in range(0, n) :
    s = input(). split()
    t = t + int(s[0]) * int(s[1]) / 10
print(format(t, ".2f"))