inp = require('fs').readFileSync('/dev/stdin', 'utf8').trim().split('\n')
N = inp.shift() * 1
X1 = Math.floor(N / 1.08)
X2 = Math.ceil(N / 1.08)
f = 0
for (i=X1;i<=X2;i++) {
    if (N == Math.floor(i * 1.08)) {
        X = i
        f = 1
        break;
    }
}
console.log(f ? X : ':(')
